Well, I do not believe $260 million went to Firefox development. I would be surprised if the majority of that went to other non-Firefox projects like:
Various AI initiatives (Mozilla.ai, Orbit, etc.)
Mozilla VPN
Mozilla Monitor
Firefox Relay
Fakespot
Mozilla Social
Mozilla Hubs
... just to name a few.
Most of these projects are open source. Anyone can see how much more active Firefox development is.
Mozilla.ai's featured projects sounded like things Firefox's AI features would use.
Orbit was a Firefox extension. Firefox integrated its features. You considered this not Firefox development?
Mozilla VPN and Mozilla Monitor are interfaces to other companies' services. And they are non Google revenue sources.
Mozilla Social was a Mastodon instance. How much software development did you believe running a Mastodon instance required?
You forgot CEO comp: 7.000.000 in 2022[0]
[0]: https://assets.mozilla.net/annualreport/2022/mozilla-fdn-990...
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.