The feature is default since 2.0, I believe.
If I move the option to one of the menus, it will show up, but doesn't work. I finally tried re-installing Firefox, and that didn't help. I did a google search for the issue, and it looks like some other people have been having the same problem with the latest versions of Firefox, I guess I should ask about it on the mozillazine forums...
Found the solution in the mozillazine forums - http://kb.mozillazine.org/Browser.se....max_tabs_undo