Greetings! I've faced a small issue with bookmarks management recently, and admittedly had not found it yet.
Wouldn't it theoretically be simpler for bookmarks to be initially stored in a common 'root' folder, thus treating the whole bookmarks storage as a single 'file system', instead of separating between 'Device X bookmarks', 'Device Y bookmarks' and so on? Especially for Firefox Mobile, it's slightly annoying to always have to open up 'PC bookmarks' every single time I need to find a favorited link that hasn't been saved on my phone.
I believe this potential change could reduce a potential issue with duplicates when syncing, as well as improve navigation of the bookmarks list due to eliminating the necessity of first opening a specific device's bookmarks directory in case of not remembering on which device the user has created a specific bookmark. If the default setting approach seems too drastic, perhaps it could alternatively become a setting for the users to define to their preference.