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