While our Mozilla Mail tutorial focuses on Windows, keep in mind that Mozilla Mail is a "cross-platform" email client: it runs and operates the same way on Windows, Apple's Mac OS X and Linux.
The Mozilla Foundation also offers the standalone Thunderbird email client: see the Mozilla Thunderbird Tutorial
Mozilla Mail Extensions / SeaMonkey Mail Extensions
Mozilla "Extensions" are like mini-programs working on top of Mozilla Mail or the Mozilla browser, which extend Mozilla Mail's (or the Mozilla Suite's) basic functionality. Mozilla Mail comes without extensions, but you can download extensions for free from Mozilla's website.
Extensions for the Mozilla Suite (& Mozilla Mail - now known as SeaMonkey) 
Like Themes, some Extensions do not support all versions of Mozilla Mail. It is not dangerous to install an obsolete extension, it will simply waist your time, (by refusing to work.)
Minimize To Tray
A noteworthy Mozilla Suite/Mozilla Mail extension is "Minimize To Tray", which allows you to …well… minimize Mozilla Mail (as well as the Mozilla and FireFox browsers, or Thunderbird) to the system tray area of Windows, next to the clock.
The Minimize To Tray extension can be downloaded and installed from the Mozilla website:
https://addons.mozilla.org/en-US/firefox/addon/2110
(use the Mozilla browser.)


Email Clients Tutorials
Mozilla Mail Tutorial