hello, please make sure that you have set windows to display at 100% in the windows control panel > appearance > display. more information about that is available at http://windows.microsoft.com/en-us/windows7/make-the-text-on-your-screen-larger-or-smaller

Try to use Firefox hot keys;

<Control> + (plus) = Zoom Bigger
<Control> - (minus) = Zoom Smaller
<Control> 0 (zero) = Reset Zoom

You might want to check out;

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.