The Firefox-on-XULRunner build system is basically in place now: it can produce working builds on Windows and Linux with a couple special configure flags. I even have a set of changes that allow it to be built on tinderbox. Get your hot-off-the-press Windows build here! Note, this was built with MSVC7.1 and you’ll need to already have the runtime libs on your system.
I won’t post Linux builds because my Linux box is x86_64 and is totally hacked up anyway.