Click on View in your Firefox menu, located at the top of your screen. When the drop-down menu appears, scroll down to the option labeled Text Size. A sub-menu will now appear with three options. Click on the option labeled Decrease.
You will notice that all of the text within the web page you are viewing has gotten smaller. To decrease the size again, just repeat these steps.
Please note that you can use the following shortcut keys in lieu of clicking the menu choice: Apple and - (Minus).
