Interesting insight, thanks for that. I ran a very quick research to figure out if Firefox is using a different Engine on MacOS (which could possibly impose a security mechanism I'm not aware about), but that seems to be restricted to iOS.

I'm afraid that won't be anything I can fix, other than to note this incompatibility on the addon's store page, sorry.

P.S.: Hmm, according to AMO, >10% of all users are on MacOS, but the information doesn't go any deeper than that. I suspect a version or even a configuration issue.