Netflix says my browser needs to be updated but Firefox says its up to date

Hi there! What windows version are you running? If your using windows 10, FireFox has reached its end of support for windows 10, meaning it won't be getting anymore updates. So your FireFox isn't accutaly upto date. You can read more about EOS here.

Firefox doesn't tell sites whether it's running on Windows 10 or Windows 11 so users running recent versions on Windows 10 appear the same as anyone else, so the sites can't bug them for no reason.

Notably from the article What Windows 10 end of support means for Firefox users quoting:

"If you're a Firefox user on Windows 10, you may wonder what this means for your browser experience. You shouldn’t worry: Firefox will continue to support Windows 10 for the foreseeable future."

While I don't understand why Netflix should have any issues with ESR versions, could you just try the current release v150 in case you're on v140 ESR they don't like?

Possible trick sending different version number for those who prefer ESR, but want to appear as newer release: https://support.mozilla.org/en-US/questions/1545628

Previous Netflix errors caused by addons: https://support.mozilla.org/en-US/questions/1538965