Browsers should probably just stop sending user-agent header at all, ideally.
If anyone else was wondering why some websites and the “Alternate Player for Twitch.tv” extension stopped working in the latest Librewolf update, it’s because they changed the userAgent string from Firefox to LibreWolf and way too much shitty code is confused by it looking like firefox but then not being firefox.
You can’t stop sending it. That triggers bot detection. If you send a fake header, that doesn’t match existing fingerprints of that browser, bot detection.