So I've been using FF3.1b1 and have loved it. The speed improvements and cool new alt+tab style tab switcher have really been incredible. GMail has never loaded so quickly for me. The only thing I couldn't get working right away was Flash. So I dug around a bit and realized it was a matter of copying a file into the plugins folder of my beta installation.
Head on over to Adobe's download site and choose the tar file to download. Extract the files anywhere you'd like and simply copy libflashplayer.so to /path/to/firefox/plugins (for me it was ~/firefox_beta/plugins but default installs are to ~/usr/lib/firefox/plugins in Ubuntu and ~/firefox/plugins for most distros). Go ahead and restart Firefox and Flash will now be working properly.