I think the primary factor was simply that Google switched from promoting Firefox to promoting Chrome after its release in late 2008, so once any inertia in the public mind wore off, they couldn't keep up. The decline in IE continued, but it started going to a different place.
The other dominant thing in that time frame was the rise of mobile. Mozilla didn't control a mobile platform, so they were second place out of the gate on Android in a way they never were on Windows where both Chrome and Firefox had to be downloaded manually. They also didn't have a well-developed account ecosystem, so as syncing between desktop and mobile browsers became a thing, that was another nail.
The trend line seems pretty unchanged since then.
It is possible that if Brendan Eich stayed, some other people (developers passionate for gay rights etc) would leave. But the disastrous plunge in market share that was brought about by the current vanilla leadership is, IMHO, a magnitude worse than what that controversy would be able to bring.
For an analogy: Tim Cook is a great manager for Apple, but I don't believe he would be a great founder of something similarly big on the contemporary smartphone market.
Mitchell Baker is certainly passionate about things, they're just not the things which push Mozilla forward in competing with the Google-Microsoft-Apple trifecta. She is compassionate about her own remuneration, having increased it manifold [1] over what Eich took while overseeing a decline in market share by 85%. She is compassionate about "social justice", using both Mozilla-the-organisation as well as funds from the Mozilla foundation to push "racial justice" [2], "diversity and inclusion" [3], "environmental and climate justice" [4], "alphabet justice" [5] and more - just do a search for "Mozilla foundation social justice" for more examples.
Baker is not as interested in developing a browser, that is comparatively boring and does not give you any virtue brownie points so it was not that hard a decision to fire 250 people [6] to become a 'more nimble organisation' which coincidentally left more money for those other purposes she wants to pursue. Who needs developers after all when you have an internet full of volunteers?
In short, Baker needs to be fired if Mozilla is to have a chance at rising again like the Fenix or Firebird it once tried to name their browser after but which names got shot down due to trademark conflicts.
[1] https://en.wikipedia.org/wiki/Mitchell_Baker#Negative_salary...
[2] https://blog.mozilla.org/en/mozilla/leadership/mozilla-racia...
[3] https://www.mozilla.org/en-US/diversity/2021/what-we-build/
[4] https://foundation.mozilla.org/en/blog/intersections-of-digi...
[5] https://blog.mozilla.org/en/mozilla/mozilla-supports-lgbt-eq...
[6] https://www.techspot.com/news/86331-mozilla-laying-off-aroun...
But not good enough. Firefox today has not much benefit over chrome&clones. They once had them, but sold them all over the years for whatever nonsical reasons they brought up. And they also lost trust from multiple little stupid actions, driving even more people away.
At the end, I would say they simply lost because of poor management.
> So why are users switching to Chrome and clones?
I still use Firefox as my main browser since the beginning, but for some years now I also use Chromium and sometimes even chrome as a second browser, for cases where compatibility is necessary. And I guess there are many people like that, except that have no real reason to continue using Firefox, or never start in the first place.
> Is this because when you visit Google and MS properties from FF, they promote their browsers heavily
Where is this promotion everyone is talking about?
> To keep the future of open web open, Firefox must continue to exist.
Disputable.