Unfortunately not ready for prime time. I have native syncing of bookmarks turned off in Firefox, and don't use native sync in Chrome, and while this extension is relatively functional, it sometimes throws errors and then overwrites what you've just saved (bookmarks, folders of bookmarks) with outdated info it has on its server. Also puts bookmarks in the wrong places sometimes. I've lost a fair amount of research time because of this extension. I hope it improves. Haven't really come across a cross-browser bookmark syncing solution that just works, yet.
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Edit: I absolutely appreciate the developer's response. As a non-programmer, I trust the developer's statement that the loss of bookmarks is due to a Firefox bug. However, if that's the case, then I lost an entire folder of bookmarks because I dragged-and-dropped a folder in the Bookmarks Manager (the Library) from one location to another in the bookmark folder heirarchy. Or, because I selected multiple bookmarks, then used cut and paste to move them. In either case, these are basic bookmark maintenance functions--so I can't use this extension and I'm not sure it's fair to say "Firefox's native bookmarking features are fully supported" when cutting and pasting multiple bookmarks is not supported. Regardless of who is at fault.