I haven’t been able to get Flash to work on Firefox 3 - and I’m not trying to hard as I’m hoping to use Gnash sooner than later. In the meantime, I’ve used the about:config menu to disable the missing plugins notice which happens every time I visit a page that uses Flash. I’m so glad that option is available - it should be in the user preferences section!

https://bugs.launchpad.net/ubuntu/+source/firefox-3.0/+bug/200235