From the download page at addons.mozilla.org…

Allows Firefox to run a .NET ClickOnce application.
ClickOnce is an application deployment technology developed by Microsoft. (See http://msdn.microsoft.com/clickonce/)

There is also a testing guide available at: http://www.softwarepunk.com/ffclickonce/testing.html“

Download here: https://addons.mozilla.org/en-US/firefox/addon/1608

I just tested with the WPF Twitter app and it worked great.