Problem seems partially solved: I had the "Navigation bar" hidden to save screen space, if I re-enable it I've got a "back to normal screen" option when right-clicking over it (sorry if the translations from my Italian version of FireFox are not perfect...). But it could be a good idea to have a "back to normal screen" option even when right clicking anywhere in the page...

Try Full Screen context/button: https://addons.mozilla.org/firefox/addon/810

Thanks cor-el, I've found a similar tool with compact menu 2 https://addons.mozilla.org/en-US/firefox/addon/4550/

Exiting full screen is so simple. Just move your mouse to the topmost edge of the screen, preferably to the top right corner or anywhere you see the screen border is sky blue. There you do 'right click' and a menu pops up with 'Exit' link.

You can also click on "Restore Down" button which is located between the "Minimize" and "Close" botton on the top right corner.

its so simple start firefox than click restore down next to minimize and its done