Click-To-Play Plugin Telemetry

Friday, September 13th, 2013

Last week we finally turned on click-to-play plugins as the default state for all plugins except Flash in Nightly builds (which will be Firefox 26). This is a milestone in giving Firefox users control over plugins and helping protect them from being exploited via unused and unwanted plugins.

As part of this feature, we have started to measure how users interact with the click-to-play UI. Nightly users aren’t typical, so this data probably doesn’t mean much yet, but it’s nice to see it in action:


This data shows how many different kinds of plugins were present in the plugin notification UI when each user saw it. When designing the notification, we wanted to streamline the common case, which we believed was that normally there would be only one kind of plugin on a page. This telemetry data will help verify our assumption. The current Nightly data shows a single type of plugin is the most common case, but not by as much as I originally thought:

# of Plugins

Notification Count









5 or more



This data shows what user action triggered showing the plugin notification.

User Action

Notification Count

Click on in-content plugin UI


Click on location bar icon


I’m surprised that so many users are clicking on the location bar icon. That may just be inquisitive users checking what each button does, but I’ll be monitoring this as it goes up the trains to the more representative beta population. If this stays very high, then we may have a problem with distracting users with unnecessary UI.


This data shows what action users are choosing to take in the plugin notification. Note that when multiple plugins are shown in the same notification, there will be a separate action for each plugin:

User Action

Notification Count

Allow Now


Allow Always




I’m a little surprised at the distribution of “Allow Now” and “Allow Always”. When designing this UI, we expected that most users would want the “Allow Always” option, and we wanted to highlight that. But again, Nightly users are atypical and may not be a good sample. I’ll be watching this data also in beta.

I’m a wary of drawing any significant conclusions from early data, but I’m happy that we appear to be collecting the correct data and with the new telemetry dashboard it’s not hard to get at simple measurements such as this. Kudos to Taras, Mark Reid, and Chris Lonnen for getting that runing and the small daily improvements that make all our lives better.

Software Integration Is Not Evil

Monday, November 29th, 2010

Asa is wrong, and he’s being obnoxious about it to boot. What Google, and Apple, and Microsoft is doing is called software integration, and in general it’s very good that iTunes, Google Earth, and Windows Live are adding Firefox integration into their software. They are installing plugins and/or extensions using the recommended methods so that all NPAPI-capable browsers can see and load them. This is not “Google being bad”, this is Google following Mozilla’s recommendations for browser integration!

It’s true that Firefox should give users more control over integration software that is found on the system, and we’re working on prompting users whether they want to include that integration as part of Firefox. But claiming that Google, Apple, and Microsoft are somehow being evil is stupid and short-sighted. The problem, if there is one, lies entirely with Firefox, not with the software which is doing exactly what we ask of them.

Asynchronous Plugin Layer Painting

Thursday, November 18th, 2010

Firefox 4 implements a new strategy for painting windowless plugins. This should result in improved performance and responsiveness when users are visiting sites such as Hulu and Vimeo which make use of windowless Flash to render their videos.


On Windows and Linux, there are two basic modes in which plugins can render, windowless and windowed. When a windowed plugin instance is requested, Firefox creates a native widget; the operating system delivers native events, including paint requests, directly to the plugin window. This is simple, but it has a significant disadvantage: the plugin doesn’t participate in normal web layout. This means that the plugin cannot be transparent, and CSS effects such as opacity and most transformations cannot be applied to the plugin. Youtube currently mostly uses windowed plugin instances for rendering their videos.

Windowless plugin, on the other hand, do not have a native widget. Instead, events such as mouse and keyboard events, as well as requests to paint the plugin, are received by the browser and forwarded to the plugin using the NPP_HandleEvent API. Hulu and vimeo both make use of windowless plugin instances. Any Flash plugin with the wmode=”opaque” or wmode=”transparent” attribute in their <embed> or <object> tags is using windowless mode.

Asynchronous Painting

In Firefox 3.6 and earlier, every time the operating system asks the browser to paint its window, we synchronously walk the layout frames and ask each frame to paint itself. When a windowless plugin frame is asked to paint, it synthesizes a WM_PAINT event and sends it to the plugin using NPP_HandleEvent. This is straightforward, but it does involve a blocking call and process round-trip for plugins which run in a separate process.

In Firefox 4, we don’t paint the plugin directly to the screen. Instead, as soon as the plugin is visible we ask it to paint to a retained buffer (an X surface on Linux, and a shared-memory DIBSection on Windows). We retain the pixel data for the next time Firefox is asked to paint. When using D3D rendering, we can eagerly upload the plugin data to a texture, and the plugin texture is composited by the graphics card and GPU.

A Behavior Change: Opacity on Windows

On Windows, the new asynchronous painting API has one significant side effect: plugins responding to a WM_PAINT message must be aware of opacity. The device context which is passed to the plugin is backed by a DIBSection with an opacity channel. Certain Windows Drawing functions, such as the DrawText function, are not aware of opacity and will incorrectly overwrite the opacity data, leaving black splotches where transparent text was intended. Windows drawing functions such as AlphaBlend are the correct way to draw while preserving transparency information.

Most Flash and Silverlight sites work correctly with this new function, but there are a few Flash features which continue to use the old Windows APIs. This bug shows itself in current Firefox nightly builds as black splotches where text should be painted, and is being tracked by Mozilla bug 611698; we are working with Adobe to resolve this issue before Firefox 4 is released.

Testing Wanted

Although our implementation of asynchronous painting passes all of our internal tests and appears to work well in general, the web is a big place and we can’t test every page or plugin available on the web. We would really like people who develop with plugins or use plugin-intensive sites to test Firefox nightly builds and report any bugs which you see! These builds are updated to our most recent Firefox each night, so you will always have the latest and greatest features (and sometimes bugs) to experiment with.

Note to Flash Authors

If your site uses wmode=”transparent” but your Flash application is not actually transparent or translucent, you can get better performance in both Firefox and Chrome by switching to wmode=”opaque”. Please use wmode=”opaque” for content which does not need actual transparent behavior.

The Firefox Plugin Hang Detector

Wednesday, June 9th, 2010

If all goes well, Firefox 3.6.4 will be released with support for out-of-process plugins on Windows and Linux next week. As part of this project, Firefox now features a “hang detector” for plugins. The hang detector helps protect users from plugins and plugin scripts which stop responding for 10 seconds.

When Firefox makes an NPAPI call to a plugin which is being run in a separate plugin process, that call is translated into a message which is posted to the process. Firefox then waits for the response. If the plugin takes longer than 10 seconds to respond to a call, Firefox performs the following actions:

  • Stop the plugin process and collect a plugin-process “crash” minidump.
  • Collect a “crash” minidump from the browser process.
  • Terminate the plugin process.
  • Display the plugin-crashed UI, giving the user the opportunity to refresh the page and try again.

There are several reasons plugins might trigger the hang detector:

  • A plugin script (such as ActionScript run inside the Flash plugin) may be in an infinite loop or performing a very long computation.
  • The plugin itself may have a bug (such as a threading deadlock) which causes it to stop responding.
  • The plugin is not deadlocked, but is not processing events quickly enough, causing the event to linger waiting to be processed.
  • The implementation of Firefox out-of-process plugins may be causing a deadlock.

It is this last possibility that is most concerning, and we have pored over our Firefox crash stats studying the hang reports that we receive, trying to categorize the reports into one the categories above. During the long process of Firefox 3.6.4 release candidates, we have identified and fixed several “tophangs”: see bug 561817 for an example.

When hang reports were first submitted to crash-stats, it was very difficult to distinguish full crashes from hangs. It was also impossible to correlate the browser and plugin parts of a hang. Since then, the Socorro team has done an outstanding job of improving the crash-stats UI so that we can analyze hang reports. It is now possible, using the advanced query page, to search for only crash reports, only hang reports, and to limits searches to either the browser process or the plugin process. Individual hang reports are now cross-linked: see the following reports for an example:

In this particular hang-pair, it isn’t immediately clear what is causing the hang. The browser is painting, and during the process of painting a windowless Flash plugin sends the NPP_SetWindow NPAPI call. This is the hanging call. At the time the hang report was collected, the NPAPI thread of the Flash plugin is calling NPN_InvalidateRect, and inside that implementation locking a mutex. But we don’t know, looking at the stacks, whether this is a deadlock on that mutex, or whether the plugin just happened to be making that call at the time the hang detector collected its stack.

In some cases, developers may want to disable the hang detector. If you are using the Flash debugger, or if you are debugging Firefox, you can change the hang detector timeout by changing the preference dom.ipc.plugins.timeoutSecs. Setting a value of -1 disables the hang detector entirely. For more information on setting preferences, see about:config on MozillaZine.

Multi-Process Plugins on By Default

Wednesday, January 27th, 2010

Out-of-process plugins (OOPP) are now on by default in mozilla-central! Starting tomorrow morning, the mozilla-central nightly builds will load Flash and all other plugins in a separate process by default (on Windows and Linux). The Electrolysis team would love for people to test any plugins on their system, especially less-popular plugins.

Since we are moving relatively quickly with multi-process plugins, there are a few known issues to be aware of:

  • The plugin-crash UI is not finished. The current UI is just a non-localized dialog so that we can get crash reports from nightly testers. This will be changed soon!
  • On Windows, tearing/repainting issues when scrolling, bug 535295
  • On Linux, compiz effects and Flash don’t work together on some systems, bug 535612
  • On Windows, selecting “Print” option in Flash may lock up Firefox, bug 538918
  • On Windows, hulu won’t switch to full-screen mode, bug 539658
  • On Linux with GTK+-2.18 or later, GDK assertions and a fatal XError, bug 540197
  • Firefox-process crashes at NPObjWrapper_NewResolve with silverlight and sometimes Flash, bug 542263

If you discover crashes while running nightlies, please make sure you submit them, and check about:crashes for the crash ID and signature. We could use help making sure plugin-related crashes and instability are filed and tracked by searching for signatures here and filing bugs in the Core:Plug-Ins component.

If your browser hangs, you can probably recover by killing the mozilla-runtime process in the Windows task manager or via `kill` on Linux. If you are a developer with a debugger, please use the Mozilla symbol server and get stacks for both the Firefox process and the mozilla-runtime process and file a bug.

In some cases, it may be useful to the Electrolysis developers if you obtain a plugin log, which is a log of calls made between the plugin and the browser. Instructions for obtaining the log are available here.

I am very excited that we’ve made it this far, and I look forward to our next milestone release, which will backport these changes to the 1.9.2 release in preparation for Firefox Lorentz.

Flash in a separate process

If for some reason you need to disable multi-process plugins, set the pref dom.ipc.plugins.enabled to false.

Multi-Process Plugins

Tuesday, December 15th, 2009

Yesterday I landed multi-process plugin support in mozilla-central. By default, this capability is disabled, because there are still some serious bugs. But if you are willing to suffer some temporary instability, we could really use some help testing Minefield nightlies with out-of-process plugins (OOPP).

Currently only Windows and Linux support multi-process plugins: mac support requires additional work. To turn OOPP on, visit about:config, find the pref dom.ipc.plugins.enabled, set it to true, and restart your browser. Please report any crashes or instability in bugzilla: product “Core” component “Plug-Ins”. Where possible, please be as detailed as possible in bug reports:

  • Operating system: please be specific about Windows versions, since Windows XP and Windows Vista deliver some Windows events differently;
  • Page visited;
  • Plugin data from about:plugins;
  • Whether turning IPC off fixes the problem (Note: flipping the pref usually requires restarting the browser to take effect).

There is one major known bug right now: any plugin which is installed in a path with spaces fails to load. On Windows, this affects almost everything except Flash. I hope to have this fixed in tomorrow’s nightly. There is a tracking bug for all the known issues which prevent us from turning on OOPP by default.

Please direct any questions about this work to the discussion list.