Should Mozilla be trusted?

Not to be a doomer, but it feels like the skill floor to protecting my privacy is unbearably high and getting higher. Does anyone have a good resource about it?

No, mostly because the main tenet of data security is that nobody should ever be trusted - not fully, at least.

Aren't we supposed to be checking the code?

Just make sure that when you uncheck all telemetry and don't use an account, they don't send your personal data. Its open source so it should be verifiable. You don't need to "trust" them if there's no data being sent in the first place.