Currently in Thunderbird (Windows) there is an option to show the tray icon, but only when the app is minimized.
It would be awesome to have an option to *always* show that icon - regardless of the application state.
Lots of good apps have such an option and it helps productivity a lot - ability to quickly access your mail program, even when you have a lot of other apps open. Saves taskbar space too.
Note: this is mostly for the users who still stick to the option to show "full taskbar labels" vs when only the icons are shown on the taskbar - so "pinning" would not really work.