Just went to the windows option to auto hide the task bar and turned it on. Moved the mouse back and forth over the task bar area and it appears and disappears over both Firefox and Thunderbird (different monitors) as the mouse comes an goes. So I am probably not much help as I can not reproduce the issue.

Using Windows 64 bit and Thunderbird 60, Firefox daily 63.01a

Have you tried restarting with add-ons disabled from the help menu. It might be an add-on that is the problem.

I did what you said and started in safe mode with add ons disabled.. But what I figured out is that when you first start thunderbird up the task bar doesnt pop up auto when It goes down.. BUT if I minimize thunderbird and then open it back up again it works fine.. BUT if I minimize again and then open up again it doesnt work again.. and process is like that continuously.. Now im running Windows 5=64 bit buy my thunderbird is 32 bit .. Do they have a 64 bit tb and I downloaded the incorrect one.. This was very helpful Thanks

There is no "release" version of Thunderbird 64, it is still technically in development.

well im running version 60 in 32 bit.. if that helps