I would see whether the problem still occurs if you start in troubleshoot mode. https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems As you can see from the first article, an add-on is not necessarily the cause of the problem. If the problem persists, you could try a new profile. https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles You can reset a preference which may help. In a new tab, enter about:config in the address bar and search for Pixels. Change the value of the following preference to 1.0 or more (I suggest 1.5); you can change in increments of .1 or .5. The default setting of -1.0 sets the size to your system settings so it should work. '''''layout.css.devPixelsPerPx''''' Finally, you can set user interface font sizes in userChrome.css if it's necessary and you are willing to try that.