Here is what navigator.userAgent of a current Firefox Nightly build reports on MacOS Big Sur:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.16; rv:86.0) Gecko/20100101 Firefox/86.0

Instead it should be:

Mozilla/5.0 (Macintosh; Intel Mac OS X 11.1; rv:86.0) Gecko/20100101 Firefox/86.0

The user agent of Firefox reports 10.16 on Intel and 11.0 or 11.1 (I am not sure) on ARM devices. Please also note that even Safari does not report 11.1 as OS version in the user agent, Safari on macOS 11.1 reports 10.15.6. See also bug 1679929.

Note that Apple's browser, Safari, reports on the user-agent string the OS version as "10.16" on Big Sur. I can't speak to exactly why this is done, but it seems likely related to OS version checker JS in the wild. Many of these scripts assume the "10." is idempotent and instead determine if the OS is compatible by examining the minor number. With these flawed OS checkers a "11.0" macOS is considered older than "10.15" and lead to web site compatibility (e.g. preventing the download of an application due to the OS being "too old").

Safari on an M1 MBPro running Big Sur 11.2 beta:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.3 Safari/605.1.15

Firefox nightly 86.0a1 (2020-12-27):
Mozilla/5.0 (Macintosh; Intel Mac OS X 11.2; rv:86.0) Gecko/20100101 Firefox/86.0

The UA Firefox is presenting will prevent EA's Origin web site from allowing download of the Origin client.

https://www.origin.com/usa/en-us/store/download

Yes, that's why we have bug 1679929, which I wasn't aware of.

Interestingly Chrome reports Mozilla/5.0 (Macintosh; Intel Mac OS X 11_1_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36 and also has problems with downloads from that site. As it looks like the world isn't ready yet for a major version bump, and the OS version should indeed be capped as bug 1679929 suggests.

Can this bug be closed in favor of bug 1679929? It seems prudent to have any further discussion in bug 1679929.

Sure, fine with me.