You can quickly change the size of the Firefox window by clicking a bookmarklet. Reader Domen writes in and explains how to set up the bookmarklet, which is perfect for users with extra screen estate. Create an empty bookmark in your Bookmarks Toolbar and enter the following as the location:
javascript:window.resizeTo(1280,1050);
Substitute 1280 and 1050 with values appropriate for your monitor size. Clicking the bookmark will instantly resize your window. You will need to make sure JavaScript is enabled in the Firefox preferences. Thanks, Domen!