Rated 4 out of 5 stars
Should be built-in with Thunderbird. My e-mail application runs the whole day and should not take up space on my taskbar.
But everytime I update other plugins, this one gets disabled automatically and Thunderbird says that "a compatibility update was installed" and re-enables MinimizeToTray after a second restart. The version number does not increase - it's possibly a problem with my Thunderbird installation.