I'm using both every day. Firefox has better adblocking and better integration of tree style tabs. Both provided via extensions. I also prefer Firefox Sync to whatever Google has, though given Mozilla's trajectory it'll be a toss up sooner or later.

Right ok, to me extensions don't count as Firefox itself having better UI, but sure.

Support for ad blocking is by far the largest reason. Containers are also hard to imagine google allowing in chrome.

I hide most of the UI in either browser so your concern just doesn't make much sense to me.

What concern? You mentioned Firefox having better UI, I was asking how you think the two differ - ad blocking and containers are fair points, it wasn't quite what I was thinking of as UI, but fair enough.