Interesting question... How would I file a complaint against Mozilla?
Their privacy policy is only available in English and lists Mozilla Corporation in Mountain View as the legal entity.
They probably don't conduct many business activities in the EU.
However, it looks like they are providing a website for "data subject access requests" which means they might fall under EU regulation as it sounds like a GDPR compliance thing.
Maybe it is as simple as nobody having filed a complaint with a data protection agency yet.