Firefox scores well because it uses a copyleft license and the ability to relicense contributions remains with the original authors. Mozilla can't unilaterally relicense the Firefox code base as they haven't been granted that ability by the contributors. The copyleft license means they can't slap a new license on top (like a permissive license allows).
The rating criteria was designed to consider legal facators, like license terms and CLA, so concerns like Mozilla buying an ad company aren't factored in. Those concerns feel more subjective to me, but are certainly valid.
https://alexsci.com/relicensing-monitor/projects/firefox/