In Firefox, in the menu (press Alt if it's not visible by default): View > Zoom > Zoom Text Only and then zoom in/out as you desire. The setting is unfortuna

In Firefox, in the menu (press Alt if it's not visible by default): View > Zoom > Zoom Text Only and then zoom in/out as you desire. The setting is unfortunately global, so you can't zoom in the page on some sites and zoom in the text on others. (You can toggle the setting, though, and the "zoom amount" is remembered per-site, irrespective of toggling.) There are probably also some extensions that would be more flexible and work on more browsers. (I'm pretty sure that Vimperator (RIP) had an option to separately change text size and overall zoom.)