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Did you reuse existing profiles on both versions, or in either case did you start a completely fresh profile? If you haven't yet tested with a newly-created profile (which doesn't require deleting your existing profile), then you need to do so in order to triage whether the problem is with Firefox-by-default or Firefox-and-your-profile.

Since it exists in both versions of FF, could you have anything installed which could be interfering with FF's communication with the rest of the internet? Are you on a network with a router which is able to modify your traffic (requires custom SSL certificate) (long shot)?

I'd try installing your version of FF Nightly on another macOS computer to rule out that possibility. Maybe it's not a config issue but something to do with Nightly which is affecting your other, release FF