Firefox sync encrypts your data with your password (it's not exactly like that but that's the gist of it) and Mozilla has zero access to it, none at all. I see no reason not to trust them with that. It's a lot easier and _safer_ to use their service then to set up your own.
Why safer? Because with your own self-hosting, now you have to have an extra device wide open to the Internet whose safety and security you need to maintain. If it's some shared hosting, you'll have to trust your hosting provider.