WebMailCompose
If you click on a Mailto link, like the feedback link in the sidebar of this site, the browser attempts to open a desktop mail client like Mozilla Thunderbird or Outlook Express. If your primary email account is something like Gmail or Yahoo Mail this doesn't work so well because you can't actually send mail. WebMailCompose addresses this by overriding the mailto behavior in Firefox. The default solution is to open Gmail, but you can also use Hotmail, Yahoo and several niche clients.