When Firefox decided to attack the problem of market share with engineering, 2 things were happening (almost) at the same time[1]: 1) the deprecation of the old add-on engine and 2) the introduction of the new Quantum engine.
For the 1), while I understand there are some reasons in doing so (it was not a stable API as it was always breaking the extensions and security-wise), I think these problems could be managed. On the other hand, there were lots of good add-on depended on this API.
Particularly, even when Firefox was behind technically-speaking, that was the main reason I used it. Firefox replaced the old API without offering all the new interfaces with the new API. So, I lost what made Firefox really unique.
For the 2), I think it was a consequence for the 1). Firefox was finally "better" with Quantum. Of course everyone likes a responsible browser, but I actually didn't see that much difference. And there was another browser which was already really good: Chrome.
Suddenly, there was no real advantage in using Firefox anymore. Someone my say privacy. But in some corners of the Internet, it's already impossible to navigate without some privacy-wise add-on/extension, which usually work on both browsers.
Added to these, if you go deep down, security-wise there are some real technical advantages in running Chrome instead of Firefox in some OSs. For instance:
https://marc.info/?l=openbsd-misc&m=152872551609819&w=2
[1] I'm pretty sure a lot of other things were happening, but I'm in the perspective of the user here.