I installed VMWare and finally managed to get RHEL8.5 running.
As to the bug itself, I’ve tested several Firefox versions including the latest Release, ESR 91.7.0, ESR 91.6.1, ESR 91.6.0 and ESR 91.2.0 which I installed myself from https://archive.mozilla.org/pub/firefox/releases/ AND I’ve also tested the ESR 91.2.0 which came bundled with RHEL8.5.
As for the tests, with the crypto policies set to DEFAULT, all add-ons could be installed and these continued to work even after changing the policy to FUTURE, on both the browsers I manually installed and on the RHEL bundled one. For reference, I used Privacy Badger and Disconnect for this test.
Now with the policy set to FUTURE, add-ons could still be installed on the browsers I manually installed but NOT on the RHEL bundled one which threw the corrupted red error on the add-on detail page in AMO. For reference, I used Ghostery and HTTPS Everywhere for this test.
Changing the policy back to DEFAULT, allowed me to install addons unhindered on all the browsers.
So it seems, the only affected browsers are the ones which come bundled with the OS and not the ones a user might manually install afterwards.