Are you trying to install Firefox web browser on a iPhone or iPad as iOS is the mobile OS used on iPhone's and older iPad's as newer iPads have the similar iPadOS. This section is for Firefox for '''iOS'''. Or are you actually trying to install on in '''Mac OS X''' or now referred to as '''macOS'''. In your Question Details > More system details on right of your post, your User Agent appears to be Mozilla/5.0 (Macintosh; Intel '''Mac OS X 10_10_5''') AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36 So are you using the old EOL Mac OS X 10.10.5 (ten.ten.five) ? Firefox for Mac has required macOS 10.12 (ten.twelve) at minimum to run since Firefox 79.0 to current. The most current version you can run on Mac OS X 10.9, 10.10, 10.11 is 78.15.0esr. https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales can be downloaded from https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/ https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr