If the screen resolution on your computer changes when you start Firefox, making everything appear big or distorted, you likely have Window's "Run in 640 x 480 screen resolution" feature activated on your Firefox shortcut. To turn off this feature:

1. From the menu at the top of the Firefox window, select File and then select the Exit menu item.
2. Right-click on the icon you use to start Firefox and choose Properties.
3. Open the Compatibility tab.
4. Uncheck Run in 640 x 480 screen resolution. You may have also see that Compatibility mode and Run in 256 colors are selected. These can also cause problems with Firefox and you should uncheck them.
5. Click OK.

Now, when Firefox starts, your screen resolution should not change.