You may have accidentally zoomed webpage(s). Reset the page zoom on pages that cause problems. *<b>View -> Zoom -> Reset</b> (Ctrl+0/Command+0 (zero)) *https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages Firefox has a Zoom section in Settings to set the default zoom level for webpages. *Settings -> General -> Language and Appearance -> Zoom You can look at this thread: *[/questions/1384204] Since the last update, the Firefox screen does not fit my system setting(125%) <hr> If setting <b>browser.display.os-zoom-behavior = 0</b> doesn't work for you then you can create a new <b>Number</b> pref on the <b>about:config</b> page and set its value to 100. Paste <b>ui.textScaleFactor</b> in the search bar and select the <b>Number</b> radio button and click the '+' button to add the new Number pref, enter the value (100) and click the blue OK button to confirm. *about:config => ui.textScaleFactor = 100 You can open the <b>about:config</b> page via the location/address bar. On the warning page, you can click "Accept the Risk and Continue" to open about:config. *https://support.mozilla.org/en-US/kb/about-config-editor-firefox