Toggling xpinstall.signatures.required seems to be a temporary workaround. (https://lobste.rs/s/u42xyr/firefox_disables_most_extensions#c_ljgdmh). This requires a build which is either not branded, or not considered “stable” Firefox.
Nix users can get a “beta” Firefox via
nix-shell -p firefox-beta-bin, other users could get it from https://www.mozilla.org/en-US/firefox/beta/all/.
They published a Discourse post as well for updates: https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047