Use portable firefox not to let this happen. Here are scripts for it. If you like to do it yourself, however, just replace the content of firefox folder with a compiled nightybuild. Here is an instruction for it. I use portable firefox to test some addons/extensions, too. Using the portable function of apps as some kind of sandbox seems to have become a kind of trend today.

PS Indeed, the lack of addons definitely reduce the practical value of Firefox 3 but I am quite happy with Firefox 2 and there is no need to hurry.