Jabref linked identifiers DOI/URL open Thunderbird instead of Firefox ESR

[Wolfgang Engelmann]

This is a bit off topic, but since some of the people in the list are 
using Jabref as a reference manager for Lyx, I might get help.
If I click in Jabref on the linked identifiers, the DOI and/or URL 
opens, however clicking on it tries to open my thunderbird (Mail) 
instead of my Firefox. I must have done some setting since it used to 
work before, but I don't know what. I could not find a clue in the 
Jabref help.
I am using Debian and Jabref 5.1
Wolfgang