This options has been removed in version 29. Have you updated from 29 or the question is about some unofficial modifications?

Hi Alan, since Firefox 57, if you wanted to change the order of toolbars, you have needed to use style rules in a userChrome.css file. You've probably noticed that you need to update those rules every 10-15 versions and it's time again.

You can find numerous new style recipes here and on r/FirefoxCSS and elsewhere. Lots of choices.

If you need a place to start:

https://support.mozilla.org/questions/1339095

See also my reply here for new code:

• /questions/1305615 tabs below address bar - an ongoing problem not fixed !!!