You may have ad / mal-ware. Further information can be found in the [[Troubleshoot Firefox issues caused by malware]] article. Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up. ----------- '''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.