I set up multiple users on my Mom's iMac and wanted to import her firefox bookmarks to the new user. Every time I try to export/import HTML using the Organize Bookmarks Menu it never works.
Try the backup option instead and take the resulting .json file over to the other user, and I think you can either double click the file to have it populate in Firefox or go to Bookmarks->Organize Bookmarks, and then Choose the Restore->Choose File option and direct it at the migrated file. Not sure why it wouldn't be working with Import/Export HTML though...