If you unlock some of the features in Firefox, and allow scripts to move or resize existing windows. You can force a resize to various resolutions..
you would then create a bookmark, and paste the following as the "Location" of the bookmark
Code:
javascript:window.resizeTo(1024,768);window.moveTo(0,0);
That bookmarklet would resize Firefox to 1024x768, and then move the window to the top left side of the screen. you can modify those #'s to be whatever resolution. To make it work you would just click on the bookmark. If clicking doesn't work you want to enable the feature at: Tools:Options:Content:Check enable Javascript
: Click the "Advanced" button next to it, and allow existing windows to be resized.
Good luck.
Jam it back in, in the dark.