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

• ALWAYS avoid partial upgrades, lest you end up bricking your system: https://wiki.archlinux.org/title/System_maintenance#Partial_upgrades_are_unsupported
• The Arch Wiki is your best friend. You can also use it offline, take a look at wikiman: https://github.com/filiparag/wikiman
• It doesn’t hurt to have the LTS kernel installed as a backup option (assuming you use the standard kernel as your chosen default) in case you update to a newer kernel version and a driver here or there breaks. It’s happened to me on Arch a few times. One of them completely borked my internet connection, the other one would freeze any game I played via WINE/Proton because I didn’t have resize BAR enabled in the BIOS. Sometimes switching to the LTS kernel can get around these temporary hiccups, at least until the maintainers fix those issues in the next kernel version.
• The AUR is not vetted as much as the main package repositories, as it’s mostly community-made packages. Don’t install AUR packages you don’t 100% trust. Always check the PKGBUILD if you’re paranoid.