Firefox Syncβ β β β βWorks on nearly every Firefox-based browser and can mix and match. Even IronFox and LibreWolf recommend using it, as itβs encrypted end-to-end.
Floccus Bookmarks Syncβ β β β β Very flexible, syncs across many different desktop browsers and mobile devices, and for privacy it can run on your own server or encrypted on another cloud service.
Nextcloud Bookmarksβ β β β βOnline web app for managing bookmarks using your own Nextcloud server. I usually use it indirectly as the storage for syncing via Floccus.
Xmarks (Discontinued)β β β β βXmarks was a cross-browser bookmark sync service that I used for a long time to keep Chrome, Firefox, IE, and Safari on multiple computers using the same set of bookmarks. It shut down in 2018.