You can get up to about 2000 tabs per profile, though you start running into some issues after about 1000 tabs. How much RAM is available seems to be the main limitation. It's also easy to create tab groups and to move tabs between them.
The person who did the original version stopped work on it and is presumed dead in the Fukushima disaster. A group came together to redo this extension and get it to work on newer versions of Firefox.
The tab center is about the same size as the regular tab bar, but vertical. Most people have a wide screen monitor, and therefor plenty of horizontal space to spare. Vertical space comes at more of a premium. Tab Center addresses this really well.
Vertical tab bars are a must have feature. Even on modern wide-screen monitors you can only comfortably read up to about 10 open tabs. For example, if you have 18 tabs open, you get half-way down the window.