Firefox 79.0 and later requires Mac OS X 10.12 (ten.twelve) at minimum to run. For the old Mac OSX 10.9, 10.10, 10.11 users the last version to still support was Firefox '''78.15.0'''esr. Firefox 78.15.0esr.dmg https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales. https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/ An alternative is to try SeaMonkey as support for 10.9 to 10.10 has theoretically been restored for 2.53.16b1 and up. http://forums.mozillazine.org/viewtopic.php?p=14950301#p14950301 https://www.seamonkey-project.org/releases/#2.53.16 SeaMonkey is the community effort to keep the old Mozilla suite running after Mozilla stopped supporting it many years ago.