I have never once heard someone claim Firefox to be more secure. As far as I've ever heard, its sandboxing and process isolation suck pretty badly, particularly on Android, and that the project has had a hard time keeping up with web standards since their primary source of funding is Google trying to avoid getting slapped with another anti-trust.

The main things for which people favor Firefox seem to come down to better adblocker support, greater ease in totally disabling/removing telemetry and it not being directly under the control of a megacorp. Some also support it because they view Blink's dominance as creating an environment in which Google is de facto in charge of web standards.