I used Foxmarks for a long while when I was using Firefox, until Google Browser Sync came along, which has also passed. It was a great idea at the time. In its modern iteration, however, I felt like the best part about it was the inter-browser interoperability. I used it more recently for a time when I was still mostly using Chrome and then Firefox on some machines (such as BSD boxes).

All of the newer browser sync options that I am aware of are in a convenient location--built into the browser. Opera, Chrome, Firefox, et all are all getting bookmark sync options. However, I would much like to see a way to sync easily between the different browsers again. This is what made XMarks useful once it evolved past being only for Firefox. For now, I'm using all Opera, partially so I can keep everything together including my mobile devices.