The Firefox for iOS currently requires iOS 15.0 at minimum to run now since 120.0. It was iOS 14 at minimum during 112.0 to 119.0 https://www.mozilla.org/firefox/ios/126.0/system-requirements/ https://www.mozilla.org/firefox/ios/120.0/system-requirements/ https://www.mozilla.org/firefox/ios/119.0/system-requirements/ https://www.mozilla.org/firefox/ios/112.0/system-requirements/ https://www.mozilla.org/firefox/ios/111.0/system-requirements/ Perhaps if you have a old version that still supported iOS 12.x in Apple ID > Purchase History on the iPad then you may be able to install the older version. I have a old iPad mini 2nd gen with iOS 12.5.7, however since it never had a version of Firefox installed with the ID, I cannot try to see if installing a old version of Firefox from purchase history works.