This approach would be more flexible than using a hierarchical structure to categorise bookmarks. Firefox already supports this.
Avoozl Can you layout all the changes we would need to make to have tags work as you imagine them?
The dialog for editing an individual bookmark should have a field called "Tags" that accepts an optional list of tags, separated with commas.
The Edit Bookmarks window, the Bookmarks menu as well as the Bookmarks pane in the sidebar should all have a folder called "Tags" (next to "Favorites", "Bookmarks Bar" and "Bookmarks"). Upon opening that folder, the user should see all of their tags as subfolders. Opening a subfolder should reveal all the bookmarks with the corresponding tag.
You should be able to search bookmarks by their tag in the Edit Bookmarks window as well as the Bookmarks pane in the sidebar. For example, if the user has a tag called "psychology", then entering "psychology" in the search field should reveal all the bookmarks with that tag.