Finally. This is more than annoying. http://forums.opensuse.org/applications/425619-cannot-launch-url-thunderbird-3-04b-firefox.html
Config editor: network.protocol-handler.warn-external.ftp default boolean false network.protocol-handler.warn-external.http default boolean false network.protocol-handler.warn-external.https default boolean false
Change all of those to "true".
Oh, and for those of us that loathe gnome, edit /usr/lib/thunderbird/open-browser.sh to, right after
# No URL specified so set to blank url=$1 if [ -z $url ]; then url=about:blank fi
Add: exec firefox -new-tab "$url" exit
and all is good again.
mark -- "Few can believe that suffering, especially by others, is in vain. Anything that is disagreeable must surely have beneficial economic effects." - John Kenneth Galbraith