I just tried with Firefox 26 and Opus 11 beta 2, on Windows 7 x64, and it worked fine here.
Make sure Opus is NOT running as Administrator, and also not running in Compatibility Mode.
If neither of those help:
[ul][li]If you make a folder on your desktop and double-click it, does it open in Opus?
[/li]
[li]What results do you get with the two test programs in the thread mentioned above?[/li][/ul]