See this support article:
https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

Sorry, but this doesn't work. I found out the only way I can seem to do it is in VIEW>TOOLBARS>CUSTOMISE, then add spaces and this reduces the space for the address bar, you can than put in icons you need. Hope this helps someone else, as the help here is about icons and tool bars, not about the address bar.

The location bar (flex:400) and the search bar (flex:100) have a flex property and take up all available space.

You can place the mouse pointer between the location bar and the search bar and it will become a resize icon that you can drag right or left with the left mouse button pressed to change the relative width of the two bars.

You can set the max-width of the location bar and the search bar to force a maximum width of both bars.

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#urlbar-container { max-width: 400px !important; }
#search-container { max-width: 200px !important; }

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

• http://kb.mozillazine.org/Editing_configuration