So: Just use Firefox? Badger the IronFox people to build their AppImage on an older base system?
Well, the technical decisions I have made are all towards minimal maintenance, no eternal compatibility chasing, yet still being compatible with all but the most broken AppImages shipped on the web.
e.g. Avoiding musl libc; using glibc. (Based on: Void Linux. I like it and it is easy to change.)
So giving up on the IronFox AppImage as-is would mean the project has failed.