Ismael

You are not wrong but at that point it applies to any and all services out there. Companies typically send an email notifying the change in their TOS and post it in their blog/website. Depending on the change I've seen that they even say that users have a deadline to react accordingly. Firefox has its controversies but I have no reasonable suspicion that they will pull that risky move of spying on that encrypted sync process and go against their whole mission and user base.

I don't think they are lying about being against their privacy policy. Anyone can check as I just did and it seems correct.
If they were lying about the encryption I think it would have been found out by now. My impression is that Firefox users are generally more tech savvy and privacy aware so someone would have probably find out if Firefox is lying about the encryption part too. Even if that's not the case a whistleblower would have probably done it.

From their website

All your data is encrypted on our servers so we can't read it – only you can access it. We don't sell your info to advertisers because that would go against our data privacy promise.