Go to customize mode and hover your mouse pointer on the search bar. You'll notice that your mouse pointer turns into a hand, then just drag the search bar and

Go to customize mode and hover your mouse pointer on the search bar. You'll notice that your mouse pointer turns into a hand, then just drag the search bar and drop it into the center of the screen (blank space). (Tested on Firefox 55.0.2/Linux) Cheers. :)