Since everyone’s favorite Firefox extension, Tab Mix Plus, seems to be down for the count, I went looking for an extension that could replace it. The only thing I’ve found that does is Tab Kit. Since it is still in development, you’ll have to login and acknowledge you understand it is still “experimental”. I’m using it on all my computers now, and haven’t noticed any issues — other than the fact that it makes Firefox usable again for those of us who like to have >100 tabs open at a time.