For regular consumers, I'd say the biggest value proposition for Firefox on mobile, at least on Android, is the support for browser extensions such as uBlock Origin, and thus mobile ad-blocking.

As an example, there's currently no straight-forward way for the average Joe to set up ad-blocking on a mobile device (excl. DoH/DoT using PiHole, NextDNS, etc.). Many non-technical users are, however, familiar with the usage of ad-blocking extensions in their desktop browsers. They might not know the nitty gritty of how ad-blocking works, but most people seem to have one installed. Installing and using such an extension is just as seamless on Firefox for Android as it is on a regular computer.