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
This throws because the lack of tab groups.
```
Timestamp: 10/01/2014 08:19:44 PM
Error: TypeError: gBrowser.visibleTabs is undefined
Source File: chrome://nightly/content/suite.js
Line: 31
```

`gBrowser.tabs.length` would do the trick, but...