Are you running the latest version of Firefox? This was supposed to have been fixed in Firefox 24 or so (that the window would be repositioned at startup if it was offscreen).

Here are three different approaches. One of them should work!

Firefox Window "off screen" — Arrow Key Method

If you know that Firefox is off to a particular side, you can use the arrow keys to move it back on screen. You only need to move it far enough to be able to click the title bar and then you can drag it the rest of the way.

Make the Firefox tile active on the task bar and press this key combination to start moving the window:

Alt+Spacebar > m

(To clarify, that's Alt+Spacebar to open the window control menu, then the letter m to start moving the window.)

Press the arrow key for the direction you want to move the window, e.g., press the left arrow key to move the window left. Press the Enter key to complete the move.

Firefox Window "off screen" — Maximize + New Window Script Method

If Firefox is nowhere to be found but you know it's out there, try this method.

Make the Firefox tile active on the task bar and press this key combination to maximize the window:

Alt+Spacebar > x

(To clarify, that's Alt+Spacebar to open the window control menu, then the letter x to maximize the window.)

If that works, try this:

(1) Copy the following line of code to the clipboard (it's all one line...):

newwin=window.open(window.location.href, "_blank", "width=640,height=480,menubar,toolbar,personalbar");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 next to the caret (») 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.

Firefox Window "off screen" — Remove Settings File

If you can't move or maximize Firefox, you can rename or delete the settings file that stores the last window position.

(1) Open your currently active Firefox settings folder (AKA Firefox profile folder).

From the Start menu (Run box or search box), paste this into the search box and press Enter:

%APPDATA%\Mozilla\Firefox\Profiles

This should open a window showing one or more Firefox profile folders. If you have multiple folders, look for the most recent. Click into your profile folder.

(2) Switch back to Firefox and Exit completely.

(3) Wait a few moments for Firefox to finish updating files in your profile folder, then rename localstore.rdf to something else, like localstore-bad.rdf or localstore.old. Alternately, you can delete the file.

(4) You're done with your profile folder and you can restart Firefox now.

Any luck?

At first I had no luck then after playing around it worked thanks