According to Wikipedia for the iPAd Air 1st Gen the final supported OS was iOS 12.5.7 which is the same last iOS on devices like iPhone 6 and iPad mini 2nd/3rd gen. Too bad yours is not a 2nd Gen iPad Air as final supported iOS was 15.8.2. Unless you have past versions of Firefox installed in your purchase history on you Apple account to try installing, you cannot just download and install a older Firefox version like you can on desktop and Android. iOS 15.0 has been the minimum requirement since Firefox 120.0. Firefox 99.0 for iOS required iOS 13.0 at minimum. Also moved this thread from (desktop) Firefox section to the Firefox for iOS (iPhone/iPad) section.