There are a lot of dedicated anti-detect browsers, you can search for that term or fingerprint switcher, multi-accounting browsers, etc. Many of them are based on Chromium.
In my experience they're generally detectable by mismatches in various attributes compared to the "real" browser whose user agent they are spoofing (though of course, the ground truth of adversarial detection is always hard to know for sure).
I'm guessing this would be Firefox, possibly using in house extensions or userscripts designed to help further avoid fingerprinting?