A big part of the loss of users was when they lost the German market due to bad press after the Cliqz scandal in late 2017. They lost a lot of trust in Germany from that and Germany was one of Mozilla's most important markets due to how many privacy focused people there are in Germany
Not sure how something from 2017 impacts the numbers from 2019? And it seems that scandal was some months before the big switch to quantum, which should had significant more impact on the user numbers.
