Hi Rob,
all our server are Windows 7 with 2008 Webserver.
The clients were a mixed, some Vista and some Windows 7. It doesnt seem as if the OS makes any difference, since I it worked on some Vista maschines and on some it didnt.
I solved this problem after a huge amount of digging!
It seems as if this control behaves this faulty exactly as described when you have the Silverlight Plug-in 4.0.50524.0 activated in FF. I deactivated my plugin, reloaded the page and voila, the Upload button responded again!
When I actived the plugin the button again didnt work. So this is clearly the source of this problem.
One issue I still didnt figure out though, is that I had all my FF plugins activated and the Upload button used to work. Im very sure of this because I was so surprised to see it suddenly not working and I knew I havent done anything out of place with FF or the application.
While working on my system I must have triggered something which caused this problem with the silverlight plugin.
If my observations are right this means, that you will have systems where the Uploader will work even though their Silverlight Plugin in FF is activated.
Have fun with the testing Rob!
Picachuchu