Problem with Thunderbird holding settings

New to TB on Mac, ex-NS4.7 on PC user

Having problem in newsreader of TB getting View/Threads/Threads with 
Unread setting to hold from one session to next. Resets itself to All. 
Have 36 NGs I watch. Some hold. Others don't. Unable to see a pattern. 
Other settings same for all groups (ie Sort Order = Order Received, 
Messages = All)

Am I missing something, or what??

--reed