How much of this is technical versus the constant promotion on Google.com, youtube.com, etc.? It's been a _long_ time since I had significant issues supporting Firefox (and the warts I do find are just as commonly Chrome as Firefox) but using it primarily I see fairly regular upsells across Google properties and near-constant friction from Google developers choosing to implement features in a non-standard manner with user-agent checks or use of Chrome's earlier APIs rather than the standard ones (they blocked U2F/FIDO from Safari users for a long time after release, for example).
The GCP console breaks Firefox every month or two, Meet is the lone service which can't reliably do video chat, etc. That's probably not an explicit goal but it definitely highlights the risks of a single company setting priorities for so many different popular services.
This is all the same sort of environment that Chrome entered into back in 2008, versus Microsoft's Internet Explorer.
Yes, you've accurately described the challenges that Mozilla faces. If anything, it was easier for Chrome, because at that time Firefox had successfully clawed about 1/3 of the market from IE. So yes, things are harder for Mozilla today.
Which is my point! That's all just excuses for the inevitable failure of Firefox as a viable platform. It is not a viable plan for making sure Firefox survives. It's self-pitying, backwards-looking, naval gazing, and as long as Firefox's biggest proponents continue to engage in it as their main line of defense, Mozilla will never claw back any market share.
The GCP console breaks Firefox every month or two, Meet is the lone service which can't reliably do video chat, etc. That's probably not an explicit goal but it definitely highlights the risks of a single company setting priorities for so many different popular services.