Latest versions of FF on MacOS provides inaccuate User-Agent string.
I'm looking at the User-Agent strings from MacOS systems running the lastest versions of MacOS 11, 12 or 13. They appear to be reporting they are running MacOS X 10.15.7 Since all versions of MacOS X 10.* are now well past EOL, this causes problems when using this at least do some minimal testing against the system being "supported". Yes, I know it's easy to forge your UA strings, but most people do not, and it at least provides a reasonable base line of what to expect from users.
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This article may help.
https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox
In what way is the OS version important to you and not the Firefox version ? For browser support, best is to test via JavaScript whether the browser supports specific features.