Excellent Puntuado con 5 de 5 estrellas
The good news is, is this an excellent add on, and it still works fine in Firefox 25 in spite of having been abandoned by the developer.The bad news is, not only did the developer abandon it, he also took the help page down and doesn't respond to requests to put it back it. Trouble is, the default keyboard shortcut Alt+X may conflict with other extensions, or in German Firefox versions, with the Firefox shortcuts (in German Firefox, Alt+X opens the "Tools" menu).Here is how to change the NumExt shortcut to something else in a German Firefox: Type Alt+X to open Tools. Press N or use the mouse to select "NumExt (exec) from Tools - this opens a text field on the add-ons bar. In this field, type "execkey Y" to set the shortcut to "Alt+Y". Restart Firefox, and that's it.Another useful set of commands is "num" vs. "nonum" to show or hide the numbers on tabs.
Edit: @Ali Abbas, thank you so much!
BTW, just in case someone wants to fork it, the source code is available here: https://github.com/alouche/numext
You can access the guide @ http://alouche.github.io/numext/