You can change the font size on Apple iPhone iOS by doing the following: # Open Settings app found on your iPhone’s Home Screen or sub-folder on the Home Screen. # In there you’ll find a sub-section called Display & Brightness. # In that sub-section, there’s a section called Text Size. # In Text Size is where you can change the font size. The above method may vary from different types of iOS devices. This method has been performed by me which I can confirm that it works on iOS devices iPhone 7 (Product Red) and above models and their corresponding system updates. The above method only changes the text size on the keyboard part of the iPhone and doesn’t change the text size in Firefox for iOS. There are no current settings in the app that let you change font size either. Don’t forget to regularly check Firefox for updates as the updates may include new accessibility features which could make it easier for you to change or adapt the text size to your desire.