I've been using Thunderbird for a number of years and just yesterday, it has started to give me problems.
The bar in the middle with Fwd, delete and other actions, has disappeared.
I can't get a result when I click on various folders, so for each folder I appear to have to open a new window via a rt-click. It's not the way I've used it for a long time.
Snow Leopard 10.6.8, Thunderbird 16.02
Thunderbird upgrades occasionally cause odd changes. As long as you are comfortable with the potential security holes, you can downgrade to an older version to see if that restores the old behaviour.