Does that happen with every web page or only on specific pages?

Reset the page zoom on pages that cause problems: View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

See also Websites look wrong or appear differently than they should

If you got Windows 7, click start, control panel, display (make sure it's set to smaller (default), go to upper left click adjust resolution, advance settings, list all modes and then move up to the next higher resolution that's set for your computer. This solved my problem.