I know I am late. I installed firefor3 on my fedora 8 just three weeks back. I have been using FF3 on windows workstation in office ever since it was launched.
When I installed FF3 I noticed that the flash content is not working in it, though I have it installed on my system. I searched around the internet and found this way around.
ln -s /usr/lib/firefox-184.108.40.206/plugins/libflashplayer.so /usr/lib/firefox3/plugins/libflasplayer.so
You just need to link the flash module from your old firefox folder to the new folder.