Please provide details. Is the option not available? Is it that you change the default, but it changes back? Type '''about:preferences#search'''<enter> in the address bar. ------------- https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save '''Note:''' Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.