Firefox Extended Support Release (ESR) is an official version of Firefox developed for large organizations like universities and businesses that need to set up and maintain Firefox on a large scale. Firefox ESR does not come with the latest features but it has the latest security and stability fixes.
Firefox now includes downgrade protection to prevent corruption of user profile data. Starting with Firefox 68 ESR, you'll be prompted to create a new Firefox profile if you previously used a higher version of Firefox (read this to learn more).
You can download Firefox Extended Support Release for your operating system and language from the Firefox ESR download page.