Thanks so much for the details. I was able to reproduce this bug on Mac Catalina on Release 70.0 (64-bit) but not on Nightly 72.0a1 (2019-12-02) (64-bit) nor Beta 71.0 (64-bit). Which means that this bug has already been resolved and it will eventually work on future Firefox Release versions.
Firefox Nightly is made by codes and fixes made by Mozilla developers, that are compiled into a common code repository (Mozilla-central) to create a pre-release version of Firefox for testing purposes. After the code on Nightly matures, it is merged into stabilization repositories (Beta, ESR and Dev Edition) where that code will be polished until reaching the final version of Firefox (Release).
I leave you the link with the Firefox Release Calendar (https://wiki.mozilla.org/Release_Management/Calendar). When Release reaches Beta's (tomorrow) actual version (71) you will no longer encounter the problem.
I’ll mark this ticket as resolved since once the natural cycle is complete you won't have this bug; but feel free to reopen it if you experience this behaviour in the future once the migration to Release has been made.
Best regards, Flor.