We're having the same problem.  When our users upgrade to Silverlight 4, they no longer get the file selection popup.

 

We were able to reproduce this problem under Firefox 3.6.3. It worked with Silverlight 3, but not version 4.  This happened under both AjaxUploader 3.0 20091119 and the latest 3.0 20100414.

 

If you disable Silverlight on FF, it works again (Tools > Add ons > Plugins > Silverlight Plug-In 4.0 > Disable).

 

We're getting a lot of complaints, hopefully a fix can released soon.

 

Thanks!