I had thunderbird installed and didn't want to uninstall it yet so I went to:
HKEY_CLASSES_ROOT\Thunderbird.Url.mailto\shell\ope n\command
and set it to:
C:\Program Files\Mozilla Firefox\firefox.exe
http://zimbra.ncsa.uiuc.edu/zimbra/mail?view=compose
and that worked. Strange tho', I was logged in to my zimbra account in another tab, but the new tab that launched asked me to login.