When I first started using Firefox on Linux, dragging tabs was really reallyyyyyy bad but they have heavily improved it. UI just feels more polished on chrome
Sync doesn't work for me, though it seems to work for everyone else. It doesn't give me any error or a hint to what the problem might me, which is just bad UX.
Chromecasting an entire tab doesn't work, though I guess can't we can't blame Firefox for that, can we?
My unit tests take at the very least twice as long to run on Firefox
Pinned tabs occasionally just disappear and I have to create everything again. Extensions exist to prevent this but don't work with multi containers, which is honestly Firefox best feature.