Firefox Sync★★★★☆Works on nearly every Firefox-based browser and can mix and match. Even IronFox and LibreWolf recommend using it, as it’s encrypted end-to-end.
Floccus Bookmarks Sync★★★★★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 Bookmarks★★★★☆Online web app for managing bookmarks using your own Nextcloud server. I usually use it indirectly as the storage for syncing via Floccus.
Pocket (discontinued)★★★☆☆I used Pocket for ages to better manage my time reading articles. Eventually I soured on the way it’s turned into a recommendation engine. And now Mozilla’s discontinuing it. Wallabag is a decent alternative for the read-it-later aspect.
Postmarks★★★★☆A self-hosted public bookmarks/linkblogging server (think Delicious or Pinboard) that can interact with Mastodon and the rest of the Fediverse.
Wallabag★★★★☆A read-it-later type service built on open-source software that you can run yourself if you want (but don’t have to). Not as polished as Pocket, but it’s sticking around, and you know it’s not using your saved bookmarks to train a recommendation engine.
Xmarks (Discontinued)★★★★☆Xmarks 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.