As well as installing Internet Explorer 7, I did the same with Firefox 2.0 and had the fun job of upgrading all the extensions I use. As it turned out pretty all of them did so automatically, however there’s always an except to rule. Tabbrowser Extensions.

So I popped over to the author’s site to find out what the crack was and saw this rather odd message:

This extension strongly unrecommended. Tab Mix is recommended instead of this, because it is stable, light, and it covers most useful features of this.

…

This extension is strongly unrecommended by Mozilla Foundation. This extension has many many latent bugs <snip>. If you use this, you must be prepared to own huge risk maybe.

There’s a full Advantages and Disadvantages section about why you should / shouldn’t use it, which is actually an interesting read, but the end result is I’ve switched to using Tab Mix Plus in FF 2. It doesn’t do everything Tabbrowser Extensions does, but it’s close enough.