When importing existing Firefox bookmarks make sure to export those into the html file as linkding won't process the json backup file that Firefox can produce.
I am not sure if this still works but this python script [1] helped me when I moved bookmarks from Firefox to linkding long time ago. It converted folders to tags. Linkding doesn't have folders concept and rely on tabs.
Yeah, read a github issue on the topic, looks like this won't change. Tag with some spacer like `:` would be great, but the app would have to treat them differently from regular tags to be useful to me. Or maybe I'll make a pull request :-)