If you go to the Firefox Preferences, on the Content tab, there's a "Fonts & Colors" section. Click on the Advanced button for more settings. But many web sites will use their own settings.
Your best choice is probably to use an extension as cor-el recommended, but the design of some sites is messed up by zooming in. There are other extensions that handle zooming in different ways. If you want to, you might just type "zoom" into the search box at https://addons.mozilla.org and explore your options.
You can zoom in or out by holding the Ctrl key and using your mouse wheel. You can also use [Ctrl+] to zoom in and [Ctrl-] to zoom out.