Resizing oversize window
From MozillaZine Knowledge Base
Enable Javascript. Type javascript:top.resizeTo(600,400) into address bar. Hit enter.
You can also make a bookmark and place it in the bookmark toolbar with this Javascript, so you don't have to type it more than once.
Note: If your screen resolution is set to 1024 X 768 then those are the numbers you type in the brackets separated by a comma. Using your profile's screen resolution will reset the Firefox window to (almost) full screen.