How to free the rest of the web from advertising is not Mozilla's problem.
It kinds is though, the reason it exists is to ensure the internet is a healthy global public resource.
Some of the many hundreds millions of dollars they're paid annually in excess of what it costs to maintain a web browser
AFAIK Mozilla nets about $500 million a year from Google being the default search engine, which is roughly the entire budget, and is lower than what Google and Apple spend to maintain their web browsers. So your numbers seem optimistic to me.
Trying to collect data about Firefox users in order to better target ads at them
I haven't seen that happening, or at least, not "collect" in the sense of "Mozilla has data about Firefox users in order to better target ads at them". Possibly that the user's own local device has that data.
Again, Mozilla has always been an ad-funded operation. But also always without doing surveillance.
(I do 100% agree that it is a risky business to be in and that I'd hate to see it cross the line, but I'm withholding judgment until I actually see that happening.)