"Now just select the "New Button…" option from the context menu that appears by right clicking the button that you added to the toolbar. Strangely the button appears invisible after the first time you add it to the Firefox toolbar, but it is there and will allow you to create new application buttons and when you add new buttons they will not have any problem with visibility. "
http://www.inspiredgeek.com/2008/01/28/launch-external-windows-applications-directly-from-firefox/
After dragging the "Application" Button to the toolbar, indeed the buttons appears invisible. But I could right click and create a New Application button - but unfortunately, unlike what was stated above, the New Button remains invisible even after I assigned a program to it and clicking it has no effect at all.
Besides some of the comments on the website indicates others are having problems with it on Firefox 3.0.1
"I really like this extension, however it doesn’t seem to work in FF 3.0.1. It is quite frustrating.
Any word on a fix would be appreciated.
Stacey
Comment by Stacey on August 19, 2008 @ 12:15 am"
http://www.inspiredgeek.com/2008/01/28/launch-external-windows-applications-directly-from-firefox/