Okay. But when I snap the tab out of the window, the numbers on the remaining that is not updated

Damn it! I thought, I was the first to think of this!

So nice add-on. I like this.
But it is not accessible some tab that opening https://addons.mozilla.org .
I suggest that to add add-on's description about this behavior.

Not working at this moment on last version.

Perfect, does what I want and what it says

.
-> Tab Numbers

Aurait pu m'être très utile - malheureusement, les no. ne se mettent pas à jour si on ferme un onglet et ,en plus, ils sont mémorisés dans les marque-pages... ce qui ne me convient pas.

Du coup, je l'ai désinstallé
.

Probably works fine for simple cases, but gets confused when using the Simple Tab Groups add-on. Keeps counting past 9.

Very interesting! Thanks Sachin.