I can imagine that people over-customize Firefox, forget about their changes, then have bugs, finally abandon Firefox thinking it's unstable.
Even today, there are several things I used to use on a frequent basis that are missing or limited because the Quantum changes broke so many add-ons. The equivalent functionality hasn't been or in some cases can't be implemented by add-ons using the new APIs instead.
I'm a programmer. I understand the technical arguments for why they wanted to make the internal changes. But when you are dumping your #1 competitive advantage because of technical details, it's probably time to start asking some tough questions.
None of this needs to be taken on faith. There were Mozillians here and elsewhere talking at length around the time of the change about why it was necessary, and maintaining market share against Webkit browsers and especially Chrome was a major reason. I don't care enough to spend any of my Friday night digging up those references from a couple of years ago, but they shouldn't take all that much finding if you do.