Smart or Unified Folders are designed for the folders that we can anticipate in Thunderbird, namely Thunderbird's default folders and closely related others, such as variants of "Junk" and "Deleted". Since "Important" is a googlemail invention, it isn't automatically recognized by Thunderbird's Smart Folders. Why do you want the Important folder? Mine is turned off. It's just google trying to second-guess what I consider important and it's merely a subset of messages that can be seen elsewhere. Anyhow, in Unified Folders view, select and right-click the Inbox, select its Properties and I believe you'll see a dialogue where you can add the Important folder. But I predict it will just repeat some of what you find in your Inbox. (Or, of course, you could add it to any of the standard Unified folders.) Closely related to Unified Folders are Saved Search folders. Here you could create a new category of user-defined aggregating folder. I think the main snag is that it doesn't play nicely with Unified Folders view and will appear separately; mine are all under Local Folders.