I think you're probably about as dead wrong as it's possible to be on this front. First they ship millions of new LoC to Firefox on a monthly basis so the engineering efforts are open for all the world to see.

Secondly, if more than half(!?!) was spent on, say, Pocket, or Fakespot, then you would see a rise and fall in spending coinciding with the onramp and closure of those programs over their lifetimes. But in reality we have seen a steady upward march in spending, and so the interpretation that passes the sanity check is that they fold these into their existing budget with the existing development capacity they have which is variously assigned to different projects, including(!!) Firefox, where again, their annual code output is monumental and rivals Google.

Again I have to note the blizzard of contradictory accusations throughout this thread. According to one commenter the problem is they are biting off more than they can chew and need to scale back all of the excessive Firefox development they are doing (and I recall previous commenters speculating that 30+ million LoC was not evidence of their hard work but "bloat" that was excessive and that they probably could cut a lot of it out without losing functionality). But for you, the obvious problem is they're wasting all that capacity on side projects and not putting enough effort in the browser.