Mozilla was historically almost entirely funded by google for fueling their personal data collection and profiling (this only changed when firefox started to become irrelevant and mozilla had to scramble for survival and their first move was to keep the same offense but with a different actor), they refuse to block tracking and ads not to infuriate their income source (gave rise to a free software version that does this and removed the mozilla proprietary "phone home" plugin), firefox includes a remote spying mechanism called "telemetry" and punish users that take step to protect their privacy from this as demonstrated during the recent dropping of alsa support debacle.
Let's not forget their origin, the name "mozilla" stands for mosaic killer and clearly states that they aim to be dominating the web browser market. The move open source was forced and due to circumstances, firefox is a failed attempt at fixing performance issues in the mozilla suite, etc.
Mozilla and Firefox also has a deserved reputation for making far-reaching and very short-sighted decisions in a vacuum. I'd rather not trust them much either.
[1]: https://en.wikipedia.org/wiki/ICloud_leaks_of_celebrity_phot...