Added tabs counter

Now with useless resource string (why not to remove descriptions, tag names are already self-describing) and variants for SeaMonkey and Thunderbird
I would really purpose that we finally get rid of this old cruft and kill support for Firefox <4.0. What value does it bring to us to keep maintaining a version really no-one is using or better should use anymore? It only makes our code way too complex for no value. IMO it's wasted time. I'm happy to file a new issue for this for discussion.