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
`visibleTabs` only available for Gecko 2 and up, so please use `mTabs` (this doesn't exists anymore in recent Firefoxes) or `browsers` (this is still available) here for Fx 3.6.
For example:
```
  return tabbrowser.visibleTabs ? tabbrowser.visibleTabs.length : tabbrowser.browsers.length;
```