From MozillaZine Knowledge Base
Gecko 1.9 implements full-page zooming. In Firefox, the zoom level is kept on a site-specific basis; that is, it “remembers“ how much you zoomed in or out on a given website. It is possible to turn off the site-specific feature with this preference.
Possible values and their effects
Remember the zoom level on a per-site basis. (Default)
The zoom level should be applied on the current tab only.
First checked in
Has an effect in