CCleaner 4.01.4093 - ThunderBird 17.06 and Windows XP SP3.
I will uncheck SESSION as suggested. Not sure what that has to do with the Quick Toolbar - certainly not obvious!
Thanks!
I turn off the Quick Filter Toolbar in Thunderbird, since I never use it. However, whenever I run CCleaner, the Thunderbird Quick Filter toolbar re-appears. in the View/Toolbars, it remains UN-checked, even though it shows up in the interface.