As I've stated in the other comment, I mainly ment the UI. But there is another feature I like to use, which is the window-specific bookmark manager. I usually keep my browser activities separated with different windows(e.g. one for youtube documentaries and one for learning). With the Chromium manager, I can have one manager-tab per window. That's not possible with the Firefox manager. It's a small thing, but hey, it's convenient.

Then again, the bookmark manager wasn't the only reason I chose Brave over Firefox. If I remember correctly, I had three reasons why I chose Brave in the end:

The bookmark manager - Reasons are stated here and in the other comment.
Brave Ads - The privacy respecting ads make my mind feel at ease when using an adblocker, as I'm still contributing something.
This one is the weakest one: Firefox just felt slower for me, to be honest.

I would've just put up with point three, but in order to leave the first two, Firefox would have to have other features I really want tu use. And that's the problem - it doesn't have those.