The Tree Style Tab sidebar keeps coming back. It's there every time I restart Firefox. It's there when I install Firefox on a new build. It just cannot be turned off permanently, no matter how many times I click on the X to close it.

It has become an annoyance to have to get rid of it every time I use Firefox. I have searched the forum and looked through the settings many times. There must be some way to disable the sidebar once it has been enabled. If so, what is it?