F11" toggles in/out of Full Screen mode, make sure you are not in Full Screen mode in order to resize a window

Drag the edges of the window to resize, drag window by title bar to move

http://kb.mozillazine.org/Resizing_oversize_window

It is possible that the screen is too wide or too high and that scroll bars and close buttons fall off.

• Use Restore or Maximize in the right-click context menu of the Taskbar icon to set focus to the Firefox application if you do not see the Firefox window.
• Open the system menu of that Firefox window via Alt+Space and see if you can move and resize that window (use the keyboard cursor keys).

If that works then first close all other open Firefox windows and then close Firefox via "File > Exit/Quit" to save that setting.

If the above didn't help then see:

• http://kb.mozillazine.org/Corrupt_localstore.rdf