One thing I truly miss from the Fox browser is bookmark separators both in the bookmarks bar to separate folders but also in the folders to separate folders from bookmarks and to be able to separate bookmarks into their own areas as well.
A simple right click on a blank space on the bookmarks bar which would show “Add new Separator” selection and would add a draggable separator to the bar. There also needs to be separators in the folders themselves as well which would add a separator above whatever bookmark or folder you are currently hovering on.
Vivaldi I think has this. Might be a good place to start looking on how to implement in the code.. although I think they use an overlay interface to achieve this.