I mean encryption is a real thing (Edward Snowden sometimes would simply say "Encryption works" in online Q and As) and there are ways to do zero knowledge data crunching Eleven while interacting with servers. Kim DotCom famously launched Mega as a zero knowledge file host after his legal issues with with megaupload, and lots of apps and services across the internet have been doing the version of this even with online services in the ensuing decade and a half. For instance I use StandardNotes which is online but encrypted.

I have no idea the extent to which any of that speaks to the Mozilla thing in this context, and local is preferable if that's achievable. And of course you should always do due diligence on what exactly is and isn't being promised when it comes to sending your data to the cloud, it certainly is possible to do that in privacy preserving ways. It's not a doubt about the technology so much as a question of institutional credibility.