[Osiris]

The Firefox add-on Fancy Numbered Tabs replaces the red X with numbers. Replaces is probably not the best term for this because the X is still available if you hover your mouse over the number. You might wonder why numbering the tabs could be useful, let me explain.
Did you know that it is possible to use a shortcut to switch between tabs blazingly fast ? You press CTRL + n where n is the number of the tab that you want to switch to. This is not a problem if you have three or four tabs open but it surely is one if you have 17 open tabs and want to open a tab somewhere in the middle.
Numbered Tabs visualize the shortcut for you. This makes it easier and faster to use the shortcut to switch to a new tab. The add-on is therefor useful for those who do use or intend to use this shortcut.

Read More:
Fancy Numbered Tabs
via Firefox Facts