The original sin, if we can call it that, is that Firefox technically lagged behind Chrome: slower, more bugs, less secure
I only want to comment on the more bugs component because for a time it seemed like all of the sites least likely to work in Firefox were Google run. Google was choosing to exclusively code to Chrome’s latest standard. When people cannot run YouTube, Gmail, whatever, but Chrome can, not surprising when users flee.
I am not talking about website compatibility, in fact, for as far as I can tell, I had no problem with Google services using Firefox. Some degradation maybe, but not enough for me to notice.
May seem trivial, but that's with bugs like these that you lose users. I wanted my mouse to zoom, my mouse didn't zoom on Firefox (and only on Firefox), I tried Chrome, it worked, found no (technical) downside, it was faster, so I stayed with Chrome. I continued using Firefox on another PC, and I saw a lot more crashes, slowdowns, etc... on Firefox, also, a single tab could crash the entire browser (it was before Electrolysis and Quantum). Website incompatibility wasn't actually that much of a problem, it mostly affected corporate intranets, which were equal part Chrome-only and IE-only during that time.
Things have improved on the Firefox side (Quantum!), and I don't use that mouse anymore, so I am back on Firefox for most part, the ad-blocking thing pushed me back, hoping that they don't to anything crazy to push me away like they did before.
