From what I can tell, Mozilla's own Firefox feedback stats support what you're saying.
https://input.mozilla.org/en-US/?product=Firefox
It's currently showing 77% of the reports about Firefox as being 'sad', while only 23% are 'happy'. It gets even worse if Firefox OS and Firefox for Android are included, too. In that case, 86% of the reports are 'sad', and only 14% are 'happy'.
I expect disappointed users to be more likely to say something, but that's still an awfully large difference between the proportion of users who are 'happy' and those who are 'sad'. When I used Firefox for Android, I'm pretty sure it sometimes prompted me to give feedback, so it's not like only disappointed users looking to complain are being sampled.
I don't know how things work at Mozilla, but at any other software product company I've ever worked at, feedback results so out of whack would've raised a lot of eyebrows, and gotten a lot of attention. Much effort would have been put toward finding out what's wrong, and what can be done to fix it, especially if the results were consistently bad for weeks or months on end.