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
The tabbrowser implementations of SeaMonkey and Firefox are different. So different, in fact, that there is no hope of ever getting the Tab Mix Plus extension to work for SeaMonkey.

SeaMonkey, Firefox and Thunderbird are partly based on the same sources (for Gecko, Toolkit, NSS, NSPR etc.) but each of them has its own UI on top of those backends, and there are many differences between these three frontends; sometimes subtle, elusive ones; other times (as with Firefox's and SeaMonkey's tabbrowser implementations) the frontends are very different.