thunderbird opens empty firefox window when clicking on link

[Dennis Jacobfeuerborn]

Hi,
I'm using the latest Thunderbird and Firefox versions installed under
/opt which works fine but when I click a link in Thunderbird it only
opens an empty Firefox window. I created a new desktop file for the
command /opt/firefox/firefox and set that as the default browser for the
gnome shell any while the right browser is started the link isn't opened.
Does anyone have an idea why that happens?

Regards,
  Dennis