Until Google sites start deliberately breaking if you don't send this data (or your browser is known to implement any other feature intended to circumvent it), thereby destroying the market share of any browser that dares to do so.
I mention this. If Firefox were to come out and say "we're going to start spoofing this data", Google servers would start rejecting Firefox users within the week. No major browser would dare do it, not even Safari and Edge, because plenty of people are forced to use Google services for work. At best, you would have a small number of people using minor browsers and passing around patches for major browsers to spoof the data discreetly.
Firefox blocks (blocked?) [0] google analytics in incognito mode in firefox and google still pays them buckets of money. It's not the same as doing it in normal mode, but it is in that direction.
I'm pretty sure Google would never dare block Safari and start a direct war against Apple - they're even powerless to resist current privacy changes on Safari. Apple has monopoly on browsers on the most popular modern mobile platform and I don't think Google can fight that.
There is a slight difference between a monopoly and a dominant market position. The latter is only achieved by consistently being better than competitors and it is easy to lose once this is no longer the case. Google with all its current power is not in the position to dictate what all the browsers should do - and prohibiting Firefox from its servers is a) an evil move that would make people reconsider their dependency on Google services (and there are a lot of companies that would be happy to get these customers) b) Is easy to circumvent by using Chrome for Google services and Firefox/whatever for anything else.
