From MozillaZine Knowledge Base
Gecko 1.9 implements full-page zooming. Mozilla Firefox exposes this functionality in the user interface through the “View → Zoom” menu and allows zooming by holding down “Ctrl” and pressing + or - or by scrolling the mousewheel up and down.
Sometimes, however, it is more desirable to merely scale the text size, and not the layout along with it. Text scaling was the default in previous versions of Mozilla. This preference allows users to switch between the two behaviors.
Possible values and their effects
Use full-page zooming (Default)
Use font scaling
The menu option “View → Zoom → Zoom Text Only” toggles the value of this preference.
First checked in
2008-02-13 by Reed Loden (patch by Elmar Ludwig)
Has an effect in