Quality is reason. No stream is as good as a UHD copy of a movie when it comes to audio and video.
There’s an argument to be made for DRM-free digital downloads over UHD discs, though those aren’t easy to obtain legally (in case legality actually matters to the user).
No horizontal tab grouping. Tab groups on Chrome are perfect, and the Firefox tab extensions all suck in comparison.
That said, I’m still using Firefox today because the internet is unusable without a good ad blocker.