He's not asking about the User-Agent string sent to websites.
He's asking about how the browser identifies itself to the Windows operating system. According to him, other forks say simply "Firefox," which makes it impossible to run them alongside the official Firefox release.
Exact match with Firefox v129.0.1: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:129.0) Gecko/20100101 Firefox/129.0