Comment 24 for bug 794720

Ok this made the trick for me. Usually, when you install any new browser these values are overriden, and you have to set them again.
Maybe it would be a good idea if these values were automatically updated when you change your default browser using "settings > preferred applications".

sudo update-alternatives --config x-www-browser
sudo update-alternatives --config gnome-www-browser

Source: http://ubuntuforums.org/showthread.php?t=1900454
Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1138161