Go to ff menu view/text size and increase the size. You can also hold ctrl and scroll mouse wheel up.

Like Klemik said: hold the Ctrl on your keyboard and move the mousewheel up. Or you can use your keyboard: hold the Ctrl key and then press the + key until you have your desired fontsize. When you want to return to the default font size, hold the Ctrl key and press the 0 (zero) key once.

if the question is related to android see Zoom in and out of websites on Firefox for Android