Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but; some icons are locked in place.

There is also a Restore Defaults button at the bottom middle.

Note: about:customizing is no longer present in Firefox 47+ releases.

• Press Alt or F10 to bring up the toolbar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and
  select Customize.

Sorry, none of this worked for me. I'm getting so frustrated, cause when I'm in the customize firefox thing, the search bar is there! I can move it anywhere but it will not stay put anywhere! After I click on the green customize tab, it just disappears! UGGGHHH!

I finally solved it! I forgot that I didn't try refreshing firefox! Sorry to bother you guys. Thanks for being there anyway!

That was very good work. Well done.