How can I get rid of the large space above my tabs?

This image shows a very large space above my tabs. I want to throw my cursor at the top of the screen and click a tab, and this prevents that. How can I get rid of this large space?

EDIT: I have fixed this problem myself by doing a "factory reset" on Firefox, which can be done by clicking on the orange Firefox button on the top-right, then going to Help > Troubleshoot Information. In there should be a button called "Reset Firefox". This puts the tabs at the back.

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