In last versions of Firefox (at least in Firefox Web Developer Edition, which is were I had this installed) this extension has/causes a memory leak, at least with you have too many tabs. It devores all the memory available and then, the machine get frozen and I need to reboot :). It goes from 6-7 gigas of normal usage (in my PC, because the software I have usually running, of course) to all my 16GB, in 1-2 minutes. It's crazy.
I uninstalled it, because, in fact, I'm not using this extension for a while, but I just comment for you to be aware of it.
Probably, this extension not being updated since last November doesn't help at this.