Hi ltica, This is controlled by the Android settings. However within the webpages this will vary from page to page. I looked for an add on that controls fonts, but did not find anything useful. There is an Accessibility feature in the Android Settings that should work in Firefox. It is called Magnification gestures. It will let you zoom in even further to a webpage in Firefox that might not have the best font size. If all else fails, there is also Talkback that can read the page to you as well. I hope this helps.