The fact that for a long, long time the vast majority of Firefox's income has come from search engine partnerships, a category google dominates?
Also: Firefox has been rather poor about user privacy. Integrating third party stuff that's difficult to remove, like Pocket, for example.
There was the whole "Looking Glass" debacle where they dropped in a Mr. Robot promotional plugin into Firefox completely silently.
When someone posted in bugzilla about it, the project manager for the plugin made the thread employee-only. It was then changed back to public briefly, before disappearing for good, reportedly being locked so even employees can't see it:
https://bugzilla.mozilla.org/show_bug.cgi?id=1424977#c21
Ask yourself: "why is a bug files about a promotional plugin so secretive that not even employees can view it?"
BTW: Guess where that project manager used to work before she worked at Mozilla? Answer: an online advertising and analytics firm (according to her LinkedIn profile at the time.)