From our mobile browsers page:
On Android, Firefox is still less secure than Chromium-based alternatives: Mozilla’s engine, GeckoView, has yet to support site isolation or enable isolatedProcess.
That is why we recommend Brave, which also features blocking (similar to what you’d get with Firefox + uBlock Origin) as well as anti-fingerprinting privacy features, while also being more secure because of its Chromium base.
Firefox fares better on the desktop side; however, Chromium-based browsers are generally more secure on the desktop as well.
We currently recommend Tor Browser for Android if you want to use Tor, because while gecko-based browsers are not ideal from a security standpoint, it’s the only officially recommended method for Tor browsing.