At the top of the Firefox window on the menu bar, click the View menu, scroll to the Zoom sub-menu, and select Zoom Text Only.

Using your mouse:

If your mouse has a scroll wheel, press and hold Ctrl while scrolling up to decrease the text font size and down to increase it.

Using your keyboard:

·         To increase the text size, press and hold CtrlCmd while pressing +.

·         To decrease the text size, press and hold CtrlCmd while pressing -.

·         To reset the text size, press and hold CtrlCmd while pressing 0.

Minimum text size

You can configure Firefox to display all text at a minimum font size. If a web page contains text that is smaller than your defined minimum size, Firefox increases the font size of the text to this minimum.

To set a minimum text size:

• At the top of the Firefox windowOn the menu bar, click on the ToolsFirefoxEdit menu, and select Options...Preferences....
• Select the Content panel.
• In the Fonts & Colors pane, click Advanced....
• In the Minimum Font Size drop-down, select the minimum size in points in which all text must be displayed.
• Font size is measured in points. There are 72 points in an inch. The most common font sizes are 10pt and 12pt.

For more information:

http://support.mozilla.com/en-US/kb/Text+Zoom