The reason Firefox has lost market share in my opinion is because:
i) Chrome is pushed by Google.com, which has a near monopoly on search
ii) Chrome was bundled and installed alongside other software
iii) Chrome is built into Android and Chromebook
iv) Chrome has a huge advertising budget
And all of these factors combine to make Chrome feel like the default portal to the web.
Also for a period of time Chrome was undeniably a better, more stable browser. It’s only relatively recently with Electrolysis and Firefox Quantum that Firefox has regained a technical parity or superiority. And those changes depended on specifying a proper UI and Extension API which is the reason why the browser is no longer so customisable.
But Firefox Quantum only exists because of Servo, and vital parts of Servo (parallel layout in particular) have not been integrated into Firefox. It’s bizarre to see Mozilla sabotaging its long term chance at technical leadership not only by scrapping Servo but firing the engineers who built it.
I had an untechnical family member say unprompted last week they want to move away from Chrome but couldn’t use Firefox Mobile because it’s too slow. At the same moment Mozilla leadership was sacking its staff working on parallelising the browser engine.