I do not think there was a iOS version of 10.15.7 and iPadOS started at 13.1 in 2019. if iOS 10.15.7 did exist it would be rather old as my old 2nd gen iPad mini has the old iOS 12.5.7 There was however the '''macOS''' Catalina 10.15.7 https://en.wikipedia.org/wiki/MacOS_Catalina macOS 10.15 (ten.fifteen) is the minimum version required to run Firefox 116.0 and later as Firefox 79.0 to 115.0 supports macOS 10.12, 10.13, 10.14 to run.