I assume you already tried to close and re-open Firefox.

You can start Firefox with command line by defining height or width of the window. Close Firefox, press the combination key Windows + R. In the popup, enter one of the following command line, then click on OK.

• firefox.exe -height 640
• firefox.exe -width 480

Next resize your Firefox window.

Note: you can use other values than 640 and 480.

Try to open the system menu via Alt+Space and use the move and resize items to make the window fit again.

You can try to rename (or delete) the xulstore.json file in the Firefox profile folder if that didn't work.

• C:\Documents and Settings\<user>\Application Data\Mozilla\Firefox\Profiles\<profile>\

"AppData" (XP: Application Data; Mac: ~/Library; Linux: ~/.mozilla) is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders