It's also hella buggy on macOS for me, which is irritating - my browser is the only thing I need to work 100% of the time. As much as I'd like to reduce Google's browser monopoly, I consciously choose to make this one exception.
I have a smaller DPI non-retina MBP2012 and I always set the default page size to 80% in Chrome. I used two extensions for the same functionality in Firefox (Fixed Zoom and a second one that worked a bit better) but I always had issues with the sites being bigger than the viewport and having to scroll around them to see the content. Images were the biggest victim to this behaviour.
You are right - there are issues in Chrome, but I don't feel them as much in my day to day life and it counts for me.
It's a very non-function related nitpick, but it bugs me constantly so I have a hard time sticking with it.
Sync with the unchangeable default iOS browser is another one, but not a lot Mozilla can do about that.
That is the only major issue I have with Firefox. I still use it as my main browser. For one thing, container tabs are a killer feature.
I was interested in Firefox containers but quickly lost interest when I discovered Firefox Sync doesn't sync container configurations, I wasn't going to replicate them in multiple places.
These issues occurred when my computer was plugged into a charger or on battery power. Replacing the thermal compound, thoroughly cleaning the fans, and reducing the number of open tabs and windows didn’t resolve this. Chrome doesn’t appear to cause these issues on my machine, holding all other workload and plugins constant.
I really want to use Firefox but cannot rely on it currently.
(apart from the obvious recent f-up with the addon signing obviously, which—while glaring—was at least a one-off)