I remember when Chrome has started their project. People were mainly switching because: tabs crashed independently from the browser, and it was way faster than Firefox. You don't need much to get people to switch.
At the beginning, yes, and the process separation was heavily marketed so people were switching for that reason even if there was actually no material benefit that would show up in their own typical usage. But on the other side, those early switchers had no reason to stay with firefox because they weren't heavy add-on users, and both browsers were nominally open source and anti-IE.
I think the bleeding of users from Firefox over time, however, has been because of a Chrome inspired cycle of constant add-on breakages, removals of configuration options, and UI changes towards a Chrome look, with each breakage and change removing a distinction between the two browsers that would keep a user from switching. Additionally as the add-on breakages were happening, Chrome was expanding their library of add-ons, further removing distinction between the two browsers, and in some cases, such as developer tools, gaining the upper hand.
I think it's important to note that this latest firefox move was not only a move to render it incompatable with its own library of add-ons, but to adopt Chrome's library as its own. The multiprocess functionality that it is now touting was a response to Chrome's own multiprocess functionality. The UI is now completely indistinct from Chrome's.
For every add-on that didn't make it over, there's 1000 users for whom that add-on was keeping them from switching to Chrome. All of the UI manipulating add-ons represented ingrained habits that 1000 users really didn't want to change, so they were bound to firefox. At the point of this version change, upgrading firefox or moving to Chrome has exactly the same amount of friction. The only way Firefox can compete with Chrome now that they're indistinguishable is on marketing, and I'm fairly sure that Google is the biggest advertising company in the world.
I suspect that this has been intentional. Firefox is again financially dependent on Google. This was as precipitous a fall as the Elop-headed Nokia.
They can still compete on privacy (Chrome won't do that), aggressive anti-ad stance (same), future improvements in security (if they manage to move more elements like JS runtime to Rust and improve sandboxing).
For the first one they need to stop undermining their own message though (by adding weird extensions people don't want). For the last one, it will take years to complete and some more time for the trend to be noticed.
