How to only change the size of the text. Instead of changing the size of everything, you can just change the size of the text. 1  Press the Alt key to temporarily bring up the traditional Firefox menus. 2  On the menu at the top, click, then go to. 3  Select.nstead of changing the size of everything, you can just change the size of the text. 1  Press the Alt key to temporarily bring up the traditional Firefox menus. 2  On the menu at the top, click, then go to.