A "must have" for me! Rated 5 out of 5 stars
I am so spoiled by this extension that I have a very hard time using any other browser besides Firefox. I even have a difficult time using Firefox on another computer if it doesn't have this extension installed.
It does work with FF3.0b3 if you install this version: http://saber.kawaii-shoujo.net/Various/tab_mix_plus-0.3.6.0.071228.xpi
The only complaint I have with this extension is that occasionally the session manager loses my tabs. Happens rarely, but nevertheless, does happen.
Kudos to the extension creator.