The + you are talking about opens a new tab in the same window.

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.

The restore defaults allowed me to locate the "+" and put it back. It now display as a different icon. What a PITA and my fault for not shutting off the auto updates. ....thanks for your help

Glad to help. Safe Surfing.