In addition to the higher prevalence of privacy (/ad blocking) conscious users on Firefox, Mozilla has also been aggressively making it easier to enable tracker

In addition to the higher prevalence of privacy (/ad blocking) conscious users on Firefox, Mozilla has also been aggressively making it easier to enable tracker blocking natively, and turning it on by default in certain cases. This has been happening with progressive versions as they're more confident it won't break large numbers of sites. Rollout of those features does actually mean this type of data needs to be interpreted differently depending on methodology.