To me:
1. Actual features like tab groups which Firefox killed.
2. Likelihood to actually add features. Firefox used to have unique features I liked a lot, like keyword searches synced via bookmarks, but this is set for deprecation and was removed from mobile. I don't remember when Firefox last gave me a new feature.
3. Adblocking on mobile while not being Firefox, which is interestingly rare.
4. Not being developed by Mozilla. Mozilla's increasingly vehement about woke political activism, and after the tech companies booted Orange Man out of the Internet, Mozilla came out in no uncertain terms saying that they'd like politically partisan platforms to decide what I see on the net. I'd used them for years and promptly noped the fuck out. The organization may be for privacy, but they don't seem to care about other important things, or rather care about them the wrong way. I want my browser maker to make a good tool, not to tell me what to think. Mozilla feels more interested in the latter than the former.
5. Brave focuses on building an alternative foothold on the web, not just providing a browser. A lot of the other browsers' footprint comes from associated web and hardware platforms and the resulting integration conveniences. Brave has a big focus on independent revenue streams, which I like a lot even if I'm not really a fan of the crypto stuff.
Firefox lacks that to a large degree. They do have Pocket, which is good, but their userbase is essentially hostile to Pocket because they have some idiotic pipe dream about a donation-funded, pure and moral FOSS project. Meanwhile Firefox the company is moralistic, not moral, and funded by Google.