You say you've already read Librewolf's FAQ, so I can skip over what they've provided in their response.

The only possible downside I could see would be that your encrypted data is stored on Mozilla servers. Which isn't a very major downside--it's properly end-to-end-encrypted. This is mentioned both by Mozilla themselves, as well as in the Librewolf docs. This is the only downside I can see right now, but for the paranoid, it might be worth looking toward the future; who knows, maybe some day, Firefox will randomly decide to disable E2EE for Firefox sync. That could be a potential downside down the road. But I find that to be pretty unrealistic... I honestly can't see a lot of ways for Mozilla to screw this up.

If the prospect of relying on Mozilla servers still makes you uncomfortable, then you can self-host a sync server, but it's not exactly a quick setup. They do provide a Docker method of installation, at least. The sync server code is found here, along with installation instructions for self-hosting and how to connect it to Firefox/Librewolf/other derivatives: https://github.com/mozilla-services/syncstorage-rs