Major problems with Firefox include:

  - full uBlock support

  - the ability to still be themed

  - first-party isolation

...Okay, okay, I’m being too cheeky.

The common wisdom is that overall Firefox can feel bottlenecked at render and draw times (“less snappy”). That could be a result of a slower JavaScript engine (takes longer to get to drawing), or a result of poorer hardware acceleration (slower drawing), or a less optimized multiprocessing/multithreading model (more resource contention when drawing).

I honestly can't see it in the real world, but synthetic benchmark are pretty clear on that front.