I've added a screen shot showing the problem.

This sounds like an addon extension causing a conflict. Restart Thunderbird in Safe Mode 'Help' > 'Restart with Add-ons disabled'

Test the Preferences window. If it looks correct then one of your addons is an issue - maybe it needs updating. you would need to restart TB to get normal mode and then disable one addon at a time to see which one is the problem.

Thanks "Toad-Hall". I started TB in safe mode and the first extension I checked (Ad Block) was the problem. I've removed that extension & now everything works!

Appreciate your time & expertise! marcus