Firefox - How to reduce spacing between icons on the Navigation Bar

I would recommend you to edit the userChrome.css-File. Yes, the name is userChrome.css.

You can edit the padding between these icons in this css-file.

For example:

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

#nav-bar toolbarbutton { 
  padding-left: 0px !important;  
  padding-right: 0px !important;
  margin-left: 0px !important; 
  margin-right: 0px !important;  
}

Here you reduce the padding between the icons to 0px. So there is less space between the icons.

Ways to do it

• I would recommend you to use ChromeEdit to easily edit the appearance.
• You can also edit the configuration-files directly. Have a look at this website. On this website, there are many ways shown how to edit the configuration/appearance.