This is the opposite of the 'open external link from prism app in default browser' feature. I have a prism app set up for Google Docs. When I get an email notification to tell me that I have access to a new document, I want to click the link and have it open in Prism. If I get an email for say a youtube video, that should open in Firefox as normal. I guess this would need some kind of 'default browser handler', that you would set as the default browser, and then it would open the correct application based on the URL.