mailto in Firefox does not call Thunderbird new email

[Foxy]

Hi, I've got this bug. Clicking on mailto: link in Firefox does not call 
new enail in Thunderbird although Thunderbird is a default mail client 
in Preferred Applications. I do not know against what package I should 
file this bug. There is a similar bug in Dupper against 
gnome-control-centre (Bug #38730) but I do not know how what package is 
responsible for this in Xubuntu.