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
Very strange. According to the docs, [gBrowser is tabbrowser element](https://developer.mozilla.org/en-US/Add-ons/Code_snippets/Tabbed_browser#Getting_access_to_the_browser), and [it must have visibleTabs property](https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/tabbrowser).