Windows 7’s taskbar supports dynamic overlay icons. They’re useful for such things as status icons and new mail notifications. We don’t support them in Gecko yet, but I have a work-in-progress patch to add support for them.

Playing along

Grab this try build and add this restartless extension to your profile. To change the icon being displayed, go to the extension’s options and enter the URL to a 16x16 static image (ICO, PNG, JPG…). These URLs can be anything Firefox supports, including http, file and Mozilla’s own moz-icon. Interesting URLs to try out:

• chrome://browser/skin/feeds/feedIcon16.png
• moz-icon://file:///C:/Windows/explorer.exe
• moz-icon://foo.jpg
• moz-icon://stock/uac-shield
• https://www.mozilla.com/favicon.ico (“yo dawg” if this were an official Firefox build)

Update (today): forgot to add: the extension’s available on GitHub too, so grab the source.