I right-mouse-clicked and removed the menu bar and all of the toolbars in Firefox a couple of weeks ago and could not find any obvious way of restoring them. If you remove all of these toolbars and controls then the menu that appears on right mouse click is no longer accessible!
My quick fix at the time was to install Google Chrome, which I have grown to like but I needed to use HTTP Fox for something today and a quick search revealed the solution to restoring the toobars. You basically launch Firefox in safemode from the commandline with this command:
A dialog will then appear providing the option to reset toolbars and controls.
An even quicker tip is to toggle the visibility of the menu bar by pressing the ALT key! (tip from Tony Smith)