First, try to reset Firefox. Refresh Firefox - reset add-ons and settings. This simple step will eliminate many Firefox problems with only one click.

A few other basic trouble shooting things you can try is this: Run all Windows Updates, install all needed service packs, etc.

Update your graphics driver (Firefox uses your graphics card for some rendering, and an out of date graphics card driver can cause problems. Upgrade your graphics drivers to use hardware acceleration and WebGL

Update all your plugins (Flash, Java, etc.): http://www.mozilla.org/plugincheck/.

Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections.

You could try the "trick" in this thread (read down to the "simplified" one): "restore down" button makes window disappear, rather than shrink.

Edit: Actually, I'll just paste this here. The guess is that the window is offscreen, so this is a way to bring it back on screen.

(1) Copy the following line of code to the clipboard:

newwin=window.open(window.location.href);newwin.moveTo(0,0);

(2) In the maximized window, press Ctrl+Shift+k to open the web console.

(3) Press Ctrl+v to paste the line of code then press Enter to run it. A new copy of that page should open in a window positioned at the top left of the screen.

(4) Switch back to the maximized window and close it first, before doing anything in the new window.