Kmail Browser link not working

[MERLIN]

Hello,
For some time now, with Kmail, I haven't been able to 
automatically open my browser (Firefox) when clicking 
on links in emails.
Is there a way to fix this? Is it a bug?
My system is an up-to-date Debian Sid, and my Kontact 
version is 6.5.3 (25.08.3).
Philippe Merlin