Hello Gammadeus, thank you. I didn't know about the "Default Full Zoom Level" extension by alice0775. I tried to find it to test it on an old version of FF, but didn't succeed to get it (http://forums.mozillazine.org/viewtopic.php?t=659681).

On the other hand, did you notice that in latest FF's, at least on Windows and Linux, there is now a native function for zooming ?
If you hold the Ctrl key and use your mouse wheel, it zooms in or out depending on the wheel direction, and you get a zoom % "bubble" in the address bar when it is different from 100%, with a way to reset it to 100% by clicking on it.
Probably not as handy as alice0775's extension, as I guess it was able to remember the zoom % per site / URL, but always better than nothing.

Also, did you look at https://addons.mozilla.org/fr/firefox/addon/zoom-page-we/ ? I wonder if this is not close enough to what you want.