AFAIK the fappening happened through a security flaw in apple icloud API that allowed for infinite password retries without rate limiting and failure to notice and notify repeated failed attempts.
Yes it still is open source software, but that itself does not mean that it puts user interest before its main income source interest, the point here was that mozilla has a strong stance for refusing to include tracking prevention and ad blocking while marketing itself as privacy minded and user centric software. Says one thing and does another.
What other browsers do or do not is irrelevant to the criticism and trustworthiness of mozilla, but for the record AFAIK there's no telemetry in otter or arora or rekonq or konqueror or pale moon or waterfox or iron or qupzilla or uzbl. Point is free software usually comes without spying mechanisms.
Besides telemetry is only one spying thing among many others that have come and gone or are still in firefox (hello, pocket, automatic geolocation, etc.) and at times disabling them does not actually prevent the behaviour from happening. have a look at the privacy patches for firefox[1] for some examples.
You seem to have missed the point about the objective or market domination, it predates IE but the point was that the vision and priorities are wrong.
That's being said, we hold similar stances here. No trust at all for chrome and opera. A bit of non blind trust for mozilla for lack of a better alternative.
[1]: https://bbs.archlinux.org/viewtopic.php?id=202766