FaviconizeTab is one of the best Firefox tabs add-ons among Aging Tabs and Tab Mix Plus. It resizes the width of the specified tab smaller up to the size of favicon. You can setup how to resize the tab, for example with a double click, or you can add the URLs you want to resize automatically.
If you have some sites always on tabs, like Gmail and Google Calendar and so on, this add-on is really a tabs space saver. More add-on for Firefox.