Firefox – Does Firefox 3.6 support HTML 5

There's the Full Fullscreen add-on.

Doesn't restrict keyboard usage, but you can configure it to run full-screen on startup.

Yes, it is. This also works fine with the final release of Firefox 4. When you install, choose to customize the install. Then just make sure to select a different directory than where Firefox 3.6 is installed. I just did it yesterday, works fine!

If you want to be able to use both 3.6 and 4 Beta at the same time, you'll need to launch the 2nd one you open with the -no-remote command line, and specify a different user profile (can't open the same user profile twice at the same time). For more details on that, see Firefox command line arguments.

Important: This does not work when using Firefox Portable, it does not recognize the -no-remote switch.

Update: If you want to keep separate profiles for your stable and beta installations, create two different profiles. Start "<path to firefox>\firefox.exe" -ProfileManager. Your existing profile will already be there ("default"). Create a new profile (call it "testing" or whatever), then choose that.

If you want Firefox to choose a specific profile for a specific installation every time you open it, you can modify the shortcuts to specify a profile. Change it to "<path to firefox>\firefox.exe" -P "Profile Name". This is case sensitive. It will then always open that profile. You could also just uncheck the box for "don't ask at startup" and choose yourself every time you start.