Firefox is no longer supported for versions older than Mac OS 10.12.x but the older version is still available as long as you accept the risk of unpatched security vulnerabilities. You need to download an older .dmg file from an archive server, after which the procedure is the same as described in the article: * https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ * [[How to download and install Firefox on Mac]] Note: My first link is for U.S. English. If you prefer a different language/locale, use the .. link on that page to show the directory of local codes, then click into the relevant one.