Firefox is my default browser, and links within emails have been working fine until just a couple days ago. Actually, I think evening before last was when I first experienced this, and received that error message in Outlook.
It sounds like Firefox auto-"upgraded" itself and now you have a problem. You could try changing your default browser to IE, testing out a link in an e-mail, and if it works OK, changing your default browser back to Firefox and then try another e-mail link.
I don't know what to think -- I opened MSIE and made it the default, and a link in an email in Outlook opened a window in MSIE just fine.
I then went back to Firefox and made it the default browser. I tried an email link, and it's the same problem again -- no new window opened in Firefox, and I got the same error message.
I'm still Googling the problem, but appreciate any and all suggestions.
Problem solved -- told Firefox to check every time if it's the default browser, and I was then given the opportunity to make it so. Then I just unchecked the box so it won't ask me again next time.