Just have one bookmarks folder both for mobile and desktop browser, having an additional "desktop bookmarks" folder on your phone just makes the whole process of adding bookmarks that much longer, if you want them added to an overall bookmarks folder, which is the whole point of syncing devices is it not? (If anything, have a "mobile bookmarks" folder, for if you want to only add the bookmark to your mobile, and let the user choose under settings (on firefox on their mobile) where the bookmarks by default go to.