Chrome definitely started the decline in Firefox's user share, but they did significant damage to themselves with the switch to Quantum and continue to do so by repeatedly changing things for the worse. It's been years now since they killed much of their extension API by switching from XUL to WE, and they still haven't (and never will) added most of the functionality back, so you just can't do a lot of what used to be possible. Firefox was great, and drew a lot of users, because of its customizability and utility. But they got scared because they were losing users to Chrome and made the idiotic decision to try and stop the bleeding by becoming more like Chrome, which was never the issue.
Most people didn't switch to Chrome because they liked it better, but because it was the default browser on a lot of devices, and most people just use the default (and some for Chrome's faster speed, but I never found the difference to be much, certainly not worth using an otherwise inferior browser for). Firefox users are, by definition, going to be the type to specifically choose their browser instead of just using the default, and so they're going to be users that don't want to use Chrome. All Mozilla did by dumbing down Firefox and Chromifying it was drive a lot of their users away, further reducing their market share.