The F11 key switches between normal view and full screen view.

Or you can move the mouse pointer all the way to the top of the screen, right-click, and choose Exit Full Screen Mode from the little context menu.

Does that solve it?

As you've no doubt noticed, Firefox's zoom feature works on a site-by-site basis. You can zoom all sites' using this add-on: NoSquint. After setting your Global zoom level, you can still fine-tune the zoom on individual sites.

After installing NoSquint, click the % on the Add-on Bar to call up site preferences, then click the Global Settings button, then the Zooming tab.

If you don't have the Add-on Bar displayed, you can right-click the new tab button ("+") or a blank area of the tab bar to the right of it and choose Add-on Bar from the shortcut menu.

You can experiment with sizes larger than 125%, but I wouldn't go much higher than 150% unless you have a large high-resolution monitor.

Hope this helps.