You can zoom the page if you hold down the Ctrl key and rotate the mouse wheel, so make sure that you aren't doing this.

Did you try to reboot the computer?

You can check if it makes a difference if you start Firefox in Safe Mode by holding down the Shift/Options key.

Do you have the option to hide toolbars turned on?

This can also help, NoSquint {web link} NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.