In a nutshell, this is a FireFox last release issue. If Internet Explorer is not your default browser, go into the internet options page (under the gear icon in IE 7) , then go to the programs tab. Click on the 'Make Default' button, then 'Apply'. Try clicking on the link from your email message again. The message should now be gone.
Raymond @ www.raymond.cc has a GREAT detailed blog entry on this issue explaining all of the reasons and conditions.
http://www.raymond.cc/blog/archives/2008/03/25/fix-general-failure-when-open-link-or-url-from-outlook-and-windows-mail/
He offers a different, more high tech solution--however, for those of you not interested in techie-stuff per se -- try the 'Easy Button(s)" approach first! 