I had a problem on my Vista laptop with Windows Live Mail where, because I prefer to use Firefox to do my web browsing, I found I couldn't click on hyperlinks in emails. When I did, I got a "general fault" and the offending link refused to open in Firefox.
Being a full-blooded IT guy, I can never be bothered to fix things on MY computer - so I left it.
Today I was called by a customer who, amongst other issues, was having the very same problem on a Vista laptop - this time, between Outlook 2007 and Firefox! As money was involved, I actually took the time to solve it :) If you have the same sort of issue - read on dear reader!...
The solution calls for a registy fix so you will need to run REGEDIT. Go to Start - Run - and type REGEDIT and press enter. (In Vista you can click Start and type REGEDIT in the search textbox at the bottom of the Start menu.)
In regedit you will need to navigate to HKEY_CLASSES_ROOT\FirefoxURL\shell\open\ddeexec
Once there you will see a name on the right hand side of "(default)" - double click this item.
You will then see a value of ""%1?,,0,0,,,,. " in a text box - delete this value (so the box is cleared) then click ok. Close the registry editor, and reboot your computer (just to be sure!) - next time you click on a link in an email it SHOULD open in Firefox just fine!!!