UI elements such as context menus and dialogs in Thunderbird could be smarter if they display information that is more contextually relevant.
For example, expanding the “Move To” context menu for an email message has a first tier of selecting an identity, then the destination folder(s) within. This interface and interaction could be simplified by immediately presenting folders associated with the identity for the email message, then the other identities as alternative options. By removing this tier of selection, users can avoid having to associate the email with the identity it came from, and also the time with selecting the extra tier by default (while still providing the option to move between identities). I've mocked this example up in the attached image.
This same concept could be applied elsewhere, such as the “Copy To” menu.