Could you elaborate why exactly?
Sure,
1) Brave has the best built-in ad blocker, so there's no need to rely on third-party extensions like uBlock Origin for Firefox or Kiwi (Chromium). Vivaldi and Cromite have weak built-in ad blockers. With Brave, there are no concerns about MV3 limitations or the update approval policy of the extension store.
2) Ability to play YouTube videos in the background out of the box.
3) Best out-of-the-box privacy settings with lots of flexibility in terms of how you want to configure them.
An open-source privacy audit of popular web browsers.
privacytests.org
These browsers are what we currently recommend for standard/non-anonymous internet browsing on your phone.
www.privacyguides.org
4) Brave, being based on Chromium, offers the best security on Android and compatibility with most sites. Firefox lacks per-site process isolation and does not use proper Android sandboxing. Progress on these security issues is too slow. Their team strength is not adequate.
divestos.org
You will need to add many extensions to make Firefox comparable to Brave, which will create a unique fingerprint for you (weakening your privacy) instead of blending you with other users. Additionally, using many extensions will increase the attack surface (increasing the risk of being exploited).
5) Brave and other Chromium-based browsers are more performant and battery-efficient on Android compared to Firefox.
6) Brave's tab handling is also superior to Firefox's due to tab groups.
Honestly, I don't see any point in using Firefox on Android right now. Multiple companies (Google, Microsoft, Brave, Opera, Vivaldi, etc.) are working towards improving the Chromium base, which automatically benefits all Chromium browsers. Additionally, Brave is open source. The Google blobs used in Brave are only for essential functions, posing no privacy concerns, and you can disable all crypto-related features.