Mozilla 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.
Even if the SeaMonkey suite doesn't benefit from the wide publicity enjoyed by the Firefox web browser since its early days, when it changed its name from "Phoenix" and reached version 1, most of the extensions ("add-ons") developed for Firefox also work with SeaMonkey: this makes SeaMonkey the most full-featured Internet Suite currently available!
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. (We have covered a related add-on in its own tutorial, in more depth: see the ThunderTray extension for Thunderbird.)
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.)