It slowed my Firefox considerably. Whenever I opened a new tab, closed one there would be a delay of 10-12 seconds! It is that bad. Even loading a page at some point stops everything else.

Firefox would frequently consume 95-100% of the CPU and appear to be hung. It almost made me think to go back to IE. The time appeared to be geometrically proportional to the number of opened tabs.

Finally I uninstalled it and things are back to normal. I know of two versions of TabBrowser Extensions. I am hesitant to try them now.

If you are looking for a good Firefox experience then stay away from TabMix .2 Extension.

Update: I am using TabMix Plus 0.2.4 for 15 days now without any problems. If you are looking for TabMix functionality try TabMix Plus.