It's pretty close to being the same thing. You're downloading Firefox at some point and not verifying the binaries you get match the source.
Unless Firefox provides fully reproducible builds on your platform from an open source compiler, you have no guarantee that the binary you have is built from the public source code. You have to trust Mozilla.
Without reproducible builds, compiling the source yourself would be the way to go.
Anyway, I agree that it should be clear that this file sharing service, while convenient, essentially requires you to trust Mozilla with your data. The claim "Mozilla does not have the ability to access the content of your encrypted file..." is fragile.