I am using Firefox Sync because I use Firefox as my primary browser, but Firefox Sync completely messes up my bookmarks, creating duplicates each time I log on to one of my computer. Seems to work only for a single-computer setup, which is quite outdated nowadays.
Xmarks has been discontinued since months. Last time I tried it because the website came back online I don't know why, the server wasn't responding at all.
Manually importing bookmarks works, AGAIN, for a single-computer setup. But I will never always think about saving my bookmarks when I leave one computer, copy that somewhere (e.g., Dropbox), and restoring when logging on another computer.
It seems that the only way is now to bypass the browser's bookmarks and setup a web server with an HTML page with links, and update the page manually to add bookmarks, or get rid of all computers except one in order to use some bookmark-syncing options above.