Rated 5 out of 5 stars
Works very well since version 3.8 (there was an issue before with IMAP folder sync, but was kindly fixed by the developer). Only one minor downside of the new versions after TB68: the "do not close" window is a bit annoying (e.g. when clicking on a new mail notification, the mail does not open up because of it).This review is for a previous version of the add-on (3.4).
For most people, this works. Please take a look into the error console ('Tools'->Developer Tools') and check for messages related to the add-on. Send me the result via e-mail!
I forgot: "Illegal imap folder defined" means that you deleted or renamed the folder which you have selected as the up-/download folder. Just select a new IMAP folder in the settings!