Both Brave and Firefox are both weakly copylefted libre software (MPLv2). However both programs are culpable to privacy pitfalls and bad practices. Brave has its infamous crypto/ad scheme and firefox has google search as its default engine (among other opt out telemetry). Both have users run nonfree javascript by default.

Use firefox instead of Brave since firefox gives you more freedom on how hardened you want your web browser to be from a very low level. Theres also Librewolf for privacy and GNU Icecat for freedom.

Also package maintainers have firefox in their repos and virtually never have Brave