Hi ghaladhriel,

I'd try starting Firefox in Use Troubleshoot Mode in Firefox. If you don't have the issue while all of your add-ons, extensions, and themes are disabled, you can try adding them back in one by one until you find the culprit. You should look at the Extensions and Themes troubleshooting guide and the Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems article as well.

You could also try Creating a new profile.

Hopefully this helps!

Sometimes the Firefox window gets off screen and continues to open there. One technique you can try is the following:

While the Firefox window is active (you can judge by the Windows task bar, press Alt+Spacebar to display the system menu for the Firefox window.

You may or may not be able to see the menu, but if you do, its position in an upper or lower corner provides a clue as to where the Firefox window is lurking. If it's on the right side, tap the m key and use the left arrow to move the window to the left. If it's on the left side, tap the m key and use the right arrow to move the window to the right. Similarly, if the system menu appears toward the top of the screen, use the down arrow to move the window down, and vice versa. It may take many presses. Does that work?

If necessary, you could try deleting a settings file that stores the last location of the Firefox window. After locating your profile folder, close all Firefox windows before deleting the file. This article has more information: http://kb.mozillazine.org/Corrupt_localstore.rdf