It has nothing to do specifically with Firefox, but is all about the incentives. If someone puts a non-trivial amount of effort into developing a product, they could be motivated by:
1. Trying to make money off selling it to end users.
2. Trying to make money by selling the end users' attention to advertisers.
3. Vanity, if it's a side/hobby project.
4. Influence the audience (pushing things aligned with their values into the audience's attention span).
5. Being a loss leader of a sort. E.g., trying to grab a market share in expectations to make money later, or sell growth figures to investors.
So, specifically to web browsers:
* Chrome, the de-facto standard, is clearly sustained by option #2.
* Because Chrome is free-to-use, option #1 is not viable for any competition.
* Because web browsers are complex and require a lot of upkeep, #3 is also mostly out of the picture.
So, if Mozilla wants to stay alive, they have no other choice than to treat their end users as the merchandise. And, to differentiate themselves from Google, they will keep making bullshit claims that will have nothing to do with reality. Even worse, as long as the current approach to antitrust doesn't change, there won't be any long-term alternatives to this model.
All of this could be swiftly solved by deeming indirectly subsidized "free" products anti-competitive (e.g. cash flow coming from outside the direct users), and not allowing mergers above something like a 20% market share, but I can't see this happen anytime soon.