Try to install it directly from github: https://github.com/mozilla-mobile/fenix/releases/tag/v82.0.0-beta.3

Interesting thought.

Tried all four versions there on both tablets and got the same response on every one.

Parse Error There was a problem parsing the package.

Any other things to try? Different installer? Anything else?

Strange. There's also one on f-droid: https://f-droid.org/repo/org.mozilla.fennec_fdroid_811100.apk

Same result.

One thing I should have mentioned is that these tablets are cheap Chinese tablets. I read a lot and having a cheap tablet is a must as clumsy as I am. I drop them all the time.

those Chinese tablets may lack some Android components eg. Full Google play support. So its no surprise that you can't install Firefox although I don't know the mechanism.

Maybe you have to reinstall Google Play.

btw, can you install other browsers？