My understanding of the GDPR is that it doesn't apply if the analytics are fully anonymized, and only partially applies if the analytics are pseudonymous[1]. It's exceedingly likely that one of these cases applies, since the ID in question is tied to a single Mozilla installation, not individual user or even browser profile.
At the same time IP addresses have been ruled to be personal information. They very clearly don't uniquely identify a person, but are close enough that it doesn't matter as the GDPR is concerned. Unique identifiers in general are murky waters.
[1]: https://www.ucl.ac.uk/data-protection/guidance-staff-student...