Is that Firefox browsing window too big or too small for you? Maybe it is just right, and you would like to keep it that way? The Window Resizer add-on for Firefox lets you resize Firefox to tons of sizes (and remembers them for later!).
Here is a little more about the extension from the author:
The Browser Window Resizer is useful for testing different screen sizes. It accurately resizes your browser so you can test to see what a web page looks like in all of the standard resolution sizes. Supports the 640×480, 800×600, 1024×768, 1280×800, 1280×1024, 1600×1200 resolutions.
How do you use the tool? You can resize the current web page (once the add-on is installed) via the context menu, tool menu or handy toolbar button.