Best is to zoom the page via full page zoom and not increase the font size as that can cause issues with overlapping text on web pages. Firefox has a Zoom section in Options/Preferences to set the default zoom level for web pages. *Options/Preferences -> General -> Language and Appearance -> Zoom You can look at extensions and possibly search the Add-ons website. * https://addons.mozilla.org/firefox/addon/zoom-page-we/