First, open the Add-ons Manager and make sure it is active. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions.

If you still can't see it: Customize Firefox controls, buttons and toolbars {web link}

Type about:customizing<enter> in the address bar.

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

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

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.

I don't see the icon in the customize>overflow menu. Does this mean the app is not installed?

Did you open the Add-ons Manager? Is it listed? Is it active?

I typed "about:addons" and clicked on extensions. It is not listed.

You should contact its support for more help.