Firefox offers two zooming options: page zoom, the default, and text zoom. Page zoom enlarges the entire page as is, with all elements of a page, including graphics, expanding equally while text zoom only changes the text size.
To toggle between page and text zoom, from the menu bar, choose View › Zoom › Zoom Text Only. To zoom in or out either use the mouse wheel while holding the Ctrl key, or use Ctrl–+ and Ctrl–-. Reset the zoom factor with Ctrl–0.