Tarq57 andtxdmy1,
AV and firewall are both McAfee, in use since long before I switched to Firefox.
Call100,
Starting Firefox in safe mode and setting everything to default values cured the problem. I am 100% positive I didn't touch any settings in Firefox, the AV and the firewall, but have had other incidents, where a setting in other programs mysteriously changed. Maybe I need to clean out some stuff.
Thanks for your ideas,
brakes