The customizer window should pop up, you can edit the Firefox UI from this window. This document shows you ways to customize your Toolbar.

By default, Firefox lets you add toolbar items to either side of the address bar. Edits to your Firefox profiles userChrome.css file can make all sorts of fun and useful changes to the foxs chrome, menus, dialogs and toolbars. Press alt, you should see the old style menu bar. You can customize the Firefox Toolbar by choosing which buttons you want to display, resizing the buttons and adding or removing Toolbars. toolbar icon has really been proofed against failure (meaning I cant see anything left to do for now) and I am not able to reproduce any issue with it, trying all sort of Firefox versions (Pale Moon, SeaMonkey, FF41, Nightly) with legacy/Australis toolbar icon. To customize your toolbar, click on the menu icon and go to More tools > Customize toolbar. Whatever you do donР Р†Р вЂљРІвЂћСћt delete or change the following line (which is there url(' ) Р вЂќР вЂћll the changes that are to be made have to be added after the above mentioned line. Then restart Firefox and customize as you wish. The main toolbar background graphic is named toolbar-background.png and located in the /global/toolbar/ folder. Step 4: Open up userChrome-example.css with your favorite text editor (no Microsoft Word or word processors please). There are three main toolbars in Firefox, each that can have a separate background.

Customize firefox toolbar