I am using Vista Home Basic 64 and have Firefox as my default browser.
I using my RSS reader, FeedDemon the other day and when I went to open up the feed in another browser, I received an error message that said "This file does not have a program associated with it for performing this action. Create an association in the Set Associations control panel".

The today I tried to go to Firefox from a link in Windows Mail and I received the same error message. I think I know what the message is trying to say, so I poked around in "Default Programs" and "Associate a file type or protocol with a program", but I could not find anything.

Looks like a missing registry entry. Maybe reinstalling firefox will re-create it.