Within the new European GDPR framework, IP addresses are to be considered as personally identifiable information, so the concern is warranted. What's decisive when characterizing an information as identifiable or not is not the fact of being actually able to perform the de-anonymization of the information (e.g. via the ISP in case of an IP address), but the mere possibility of it.
Legally though Firefox would be allowed to collect this anonymous data from the user by having him/her send the data e.g. to an API endpoint they provide via IP-based communication, they would just not be allowed to associate the data with the IP address of the user submitting the data. In the end, it comes down to trusting the party that collects the data, at least if they don't perform anonymization of the IP address via other means, e.g. by passing the information through a third party proxy server.
BTW, GDPR does forbid to turn on such data collection by default (privacy by default), so they would be required to get the explicit opt-in from the user for that.
Legally though Firefox would be allowed to collect this anonymous data from the user by having him/her send the data e.g. to an API endpoint they provide via IP-based communication, they would just not be allowed to associate the data with the IP address of the user submitting the data. In the end, it comes down to trusting the party that collects the data, at least if they don't perform anonymization of the IP address via other means, e.g. by passing the information through a third party proxy server.
BTW, GDPR does forbid to turn on such data collection by default (privacy by default), so they would be required to get the explicit opt-in from the user for that.