Thunderbird Loses Ability to 'drag-n-drop' emails

["R. G. Newbury" <newbury@xxxxxxxxxxxx>]

Thunderbird on my desktop no longer allows me to drag-n-drop email from 
one folder (inbox, say) to another.

The version on the laptop allowe this. I cannot see anything in the 
Advanced preference settings which would cover this.

Can anyone help?

Geoff