Check your zoom controls.

<Control> (Mac=<Command>) + (plus) enlarge - (minus) reduce 0 (zero) restore

Type about:preferences#content<enter> in the address bar. Across from fonts and colors, press the Advanced button. On the bottom, turn on Allow Web Sites To Choose Their Own.

Fonts Information - Detected via Flash http://browserspy.dk/fonts-flash.php?detail=1

There are also add-ons that can help.

Please try NoSquint Plus {web link} Manage site-specific zoom levels and color settings

You may have accidentally zoomed web page(s). Reset the page zoom on pages that cause problems.

• View -> Zoom -> Reset (Ctrl+0/Command+0 (zero))

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

You can look at this extension to adjust the font size in the user interface.

• https://addons.mozilla.org/firefox/addon/theme-font-size-changer/