Tried today to enable 2FA for meta.discourse.org via FF 84.0.2 as already described above. Same error message.
Tried the same on Chrome 88.0.4324.96 → instant success.
Googling for The attestation format is not supported by the server. results in 4 findings, all related to the discourse source code. Seems like nobody else had this problem yet… 
Interesting: When in FF in Private Mode, 2FA login works.
Edit:
Tried again a dozen times, disabled FF addons and scriptblocker, tried private mode browsing as described above, and 2FA is working in FF.
Strange thing is: After re-enabling FF addons and scriptblocker, 2FA is still working.
Anyways, it is working, I’m happy, case closed 