Right from your toolbar in Firefox, you can choose to go full screen or restore a window from any other state with Full Screen Plus. If you like to have your web browser at a certain size for certain tasks, this add-on is a big time saver. The user interface of the add-on gives you four small buttons, four larger buttons or one large button that switches between the two full screen states.
Here is a little more about the add-on from the developer:
This extension adds functionality w.r.t. the default full screen button. It enables you:
1) to directly set the “fullscreen.autohide” user preference.
2) to switch directly from full screen mode back to a restored window.
Note that it also sets the “fullscreen.animateUp” user preference to 0 in order to avoid sluggish behavior.
You can pick up the Full Screen Plus add-on for Firefox from the Firefox Add-ons website.