If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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I have created a FOSS extension called SyncMarks to sync bookmarks and tabs. It's working with Firefox, Edge and Chromium and also on Kiwi on Android. You can sync your bookmarks independent from the browser and cross-browser. For example from Firefox to Edge or Chrome.
As a backend I would recommend my small php script which you can selhost. You only need PHP and a database like SQLite or MySQL. As fallback you can use any WebDAV share.
In addition to all good recommendations above, I also can recommend Vim Vixen or Vimium C.
These extensions enable you to control your browser with your keyboard with vim-like commands. If you are already using vim or want to use your keyboard more to comfortably navigate your system, it is a must-have :)
On Safari I use AdGuard (because uBlock Origin is not available), Bitwarden and Zotero Connector.
On Firefox uBlock Origin, Bitwarden, Zotero Connector, Cookie Autodelete, Libredirect and LocalCDN