I installed Firefox 3 beta and the addon I missed the most was Tab Mix Plus (along with IETab, but that works on the beta). Here's how I was able to have Tab Mix Plus working on Firefox 3:

1) Try downloading the latest dev build of the addon from this link.

2) If it won't install for security reasons, you can disable the security check and also disable the comparability check as outlines here.

3) If all else fails, you can install the firefox nightly tester tools addon, which will allow you to force compatibility of all non compatible extensions. if Firefox won't startup and crash , start it up in safe mode (in the start menu you'll have a firefox safe mode shortcut) and disable all addons. then enable only those who don't cause the the crash.