Resizing oversize window
From MozillaZine Knowledge Base
Enable Javascript. (tools > options > content)
Make sure scripts can resize windows. (tools > options > content > advanced JavaScript options)
Type javascript:top.resizeTo(600,400) into address bar. Hit enter.
You can also make a bookmarklet* and place it in the Bookmarks Toolbar with this Javascript, so you don't have to type it more than once. (*bookmarks that contain JavaScript content are called bookmarklets)
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.
Keywords: resize, resized, re-size, oversized, over-size