See parent. The default in FF is to shim the GA script

Also GA requires opt-in in the EU (even if some sites try to be clever and illegally make it harder to refuse than to accept) whereas presumably their first-party tracking is done in such a way consent is not necessary (e.g. sufficient anonymization that a data point can not be correlated with the user it originated from). Presumably FF users are more likely not to consent by default to begin with.