Brave is for people deep in the FOSS community and people who are bought in on web3 technologies. Firefox is for people looking to break away from Chromium-based browsers and who want great security.
Slightly, but the difference is noticeable enough to mention. Mainly due to bloat that Firefox has picked up over years of updates and the fact that Brave is built using Chromium, Google open-source browser tech that is super fast to start with.
Brave is a new technology and one that is still finding its footing. Not everyone agrees on the shape the web needs to take and Brave is right in the middle of that conversation.
No, Firefox and Brave are both very secure and have robust privacy features. Brave just enacts these features by default whereas Firefox allows you to set your level of privacy. In the end, Brave just cuts out the middle man.