Zoom text of web pages
From MozillaZine Knowledge Base
To change the text size of web pages, do any of the following
These changes will only change text sizes temporarily. "Tools -> Options -> Content -> Fonts & Colors gives you options that will stick.
Note that some web pages may display incorrectly if you increase or decrease the text size. In Firefox 3: When Text and Images are zoomed together by default, some column widths or other containers may be dependent upon image size, so enlarging both may cause page to enlarge horizontally forcing use of horizontal scrolling to see text that would normally wrap within a smaller area.
Firefox 3 also has a preference to (de)select site specific zoom and set specific zoom values. You can add more zoom values yourself. Site specific zoom options apply to the current session only.
If Firefox 3.5/3.6 is in "Private Browsing" mode then text and page zoom isn't saved to the file content-prefs.sqlite. Using "Clear Recent History" to clear the "Site Preferences" will also remove saved zoom settings.
Consider choice on View (Alt+V) > Zoom > text only, as increasing size of images may force table cells or columns to become larger and force horizontal scrolling. (in other words similar to Firefox 2)
Firefox 4.0 and above
Starting in Firefox 4, zoom toolbar buttons [-] and [+] (but not a zoom reset) can be added through Toolbar customization.
Use the "Alt" key to view the menu bar (temporarily) containing File, Edit, View, History, Bookmarks, Tools, and Help. On Windows 7 and Vista, the menu bar was hidden by default in Firefox 4 and above. These menu items are more or less available under the "Firefox" button which has the most used of the built-in Firefox menu items available in a different format. View ->Zoom is not available under the "Firefox" button. Fonts & Colors is available under the "Firefox" button -> Options (2nd column) -> Options -> Content -> Fonts & Colors.
Full-page zooming is available in SeaMonkey 2.0 and later, site-specific zoom in 2.1 and later. These settings are in Edit -> Preferences -> Appearance -> Content. Site-specific zoom values can be examined and removed in Tools -> Data Manager -> Preferences.
View -> Zoom allows to directly select a specific zoom value from the menu, which is constant up to SeaMonkey 2.4; starting with 2.5, the toolkit.zoomManager.zoomValues preference determines the stepping for both menu and Ctrl +/- keys. The list of default zoom values has been extended from 2.4 to 2.5, but the old stepping will be retained with an existing profile unless the preference is reset in about:config.
Privacy mode settings
Zooming may cause lines to extend beyond text areas, may want to wrap text, if turned off
May help on large HD monitor
Find more zoom extensions