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.

Make sure you install Firefox properly and that you do not run Firefox from within the DMG (Disk Images) file.

Open the Firefox Disk Images file and drag the Firefox application to the Applications folder on your hard drive. You shouldn't double-click the Firefox application to run it from the Disk Images, but instead drag it out of the DMG folder.

• https://support.mozilla.org/en-US/kb/install-firefox-mac