Firefox has a few special bookmarks folders which each have their own place in the browser UI:
bookmarks toolbar
bookmarks menu
synchronising collections could be solved simply by making "collections" one of these special bookmarks folders, which shows up in the collections area of the browser UI.
this folder could also have these unique restrictions to ensure they're used for collections and not for normal bookmarks:
it's not possible to create bookmarks directly in the collections folder, only in its subfolders
it's not possible to create more than 1 layer of subfolders in the collections folder
this way, collections can still show up as a separate entity in the browser UI but re‑use a lot of bookmarks code
Why introduce a feature like that if we won't be able to sync? I just bought a new phone but I can't bring my collections from the old phone. That's just absurd. If I knew collections were synchronizable I would have just used bookmarks.