I switched to Chrome years ago because it ran so much smoother than Firefox, and anecdotally I know of many others who did the same. With the switch, so did the recommendations.
Mozilla is, rather frustrating for those that lead it I'm sure, chosen largely based on principles that people found they couldn't get from Google. Things like "don't profit from me the user", "don't track me" and "don't do things in my browser without me knowing about it". These aren't things people point towards the Chrome browser with because if you expect any of this to not be done by Google, then you're kidding yourself.
Mozilla meanwhile has a pretty wide history of just... doing things that break this promise[0][1][2] (listed is mostly recent stuff, but they have been doing it since forever, going back to the forced Pocket integration).
Chrome users don't care about this stuff (since they already use a Google product), Firefox users by virtue of picking Firefox did. And when it comes to optimization, Chrome does beat Firefox pretty handily, so people started abandoning Firefox because at that point, both Mozilla and Google offer the same value proposition.
Their recent ventures into adtech are probably going to annihilate their biggest potential userbase gain, which is Google tightening the screws on adblockers and uBlock Origin in particular not playing ball with them on it. (UBOL is a joke and the UI by design makes it look like a "kiddie"/unprofessional adblocker.)
Google didn't kill Firefox (they want it alive to avoid antitrust lawsuits). Mozilla did.
[0]: https://www.theverge.com/2017/12/16/16784628/mozilla-mr-robo...
I still vividly recall the frozen UI as another tab loaded or did work. And if one tab crashed they all went with it. Annoyed me every time.
After many years[1] they sorted it out, but in my view it's clear that's what really killed their momentum, as it was such a sacrifice to stay with Firefox compared to using Chrome.
[1]: https://blog.mozilla.org/futurereleases/2016/08/02/whats-nex...
The set of features people expect from a modern browser is really big now. To their credit, the Mozilla web standards people actively fought against a lot of the scope creep like "webpages should be able to flash firmware to USB devices" or "webpages should be able to talk to MIDI keyboards" but they lost, and now those are things a web browser is expected to do.
Keeping up with all the scope creep is expensive.
Maintaining all those cloud services raises your company's operational costs a lot, you now need people on-call 24/7 to maintain everything, you need webdevs who can wrangle postgres or redis or whatever, you need security experts to make sure the cloud stack is secure end to end, etc. So I think it's also fair for people to call this cloud stuff out as a cost center for Mozilla.
More than 1% of revenue (not profit; revenue) goes straight into the pocket of the CEO.
(Especially the regulatory things that apply to me personally.)