I’ve just upgraded to firefox 3 beta and it looks pretty cool

I got my plugins working by following these steps

1. Open about:config in the address bar.
2. Right click on the list and create a boolean: extensions.checkCompatibility
3. Set its value to false.
4. Restart browser.

Firebug works in FF3 even withgout this step if you get the latest beta from here
fireclipse.xucia.com