Almost all user agent strings start with that Mozilla prefix because Mozilla made the first browser with “fancy” features, so in the early internet many websites checked for that string to determine if they should serve the nice website or the stripped down version. Later when other browsers added the features, that also had to add that to their user string so users would get the right site. Which just cemented the practice.
They are removing lots of the nice things they did like days when an the revenue goes to artists