Instead of Mulch I would recommend Cromite. It is fully open source (free of proprietary dependencies unlike Brave and Mulch), has anti-fingerprinting (unlike Mulch), and has built-in ad-blocking. Browser comparison table made by the Developer of Mulch: https://divestos.org/pages/browsers
In all honesty, I am not sure if you can sync, but I think I've seen librewolf and Mull being able to sign in to a Firefox account. I don't sync anything unless I self-host, so I have my linkwarden for all my bookmarks needs.
I've heard of a self-hosted alternative to Firefox accounts, but I would need to research that a bit.
Use Mull (made by the DivestOS developer) on mobile. It is available through the dev's f-droid repo. It is hardened Firefox mobile similar to Librewolf and supports sync because it is a Firefox mobile fork. It is also fully open source and doesnt come with proprietary dependencies (unlike standard Firefox mobile)