I've only just started using thunderbird as my email and RSS client. I have been playing around with the "folders" options, and have found that when the "Unified Folders" display option is selected, I can select any of the top-level grouping folders, e.g. the top-level "Inbox" folder that contains the inboxes of all my email addresses, and mark it as a favourite folder (unlike non-unified top-level folders, which cannot be selected as a favourite). However, these favourite unified folders do not appear in the favourite folders view. My use case is that I'd love to have some of the unified email folders in my favourites list, as I find this unified inbox view quite convenient, but I have little use for the other unified folders (many of which are either empty or are direct mirrors of items in the "all folders" view), so I don't want the entire unified view displayed, just a few of the unified items. I wouldn't mind if the result is, "you can't favourite top-level folders" or "you can't favourite unified folders", but having the option to favourite the folder with no observable affect is undesirable.