Earlier quoted context omitted.
I never left. But then I again, I replaced everything Google with paid or self-hosted alternatives about 10 years ago. From what I hear, the Google ecosystem itself can be pain on Firefox. Although I check my old Gmail once every couple of weeks and I find it to work well enough. Maybe if you live in things like Google Docs, you start getting annoyed by accumulating papercuts? No idea..
The nuisances are minor. They're enough that if you're just thinking of switching, you might notice them and not do it. But after a while, you forget about them. I couldn't tell you what they were anymore. And there are advantages as well, such as better autoplay avoidance.
about://media-engagement according to the docs (which implies engaging with the site is required, but I don’t think it is for YouTube).