userchrome.xml

<?xml version="1.0"?>

<bindings id="generalBindings"
   xmlns="http://www.mozilla.org/xbl"
   xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
   xmlns:xbl="http://www.mozilla.org/xbl">

  <binding id="js" extends="chrome://global/content/bindings/toolbarbutton.xml#menu">
    <implementation>
        <constructor><![CDATA[
            function makeRelativePathURI(name) {
              let absolutePath = Components.stack.filename;
              return absolutePath.substring(0, absolutePath.lastIndexOf("/") + 1) + name;
            }
            // The following code executes in the browser context,
            // i.e. chrome://browser/content/browser.xul
            Services.scriptloader.loadSubScript(makeRelativePathURI("userChrome.js"), window);
        ]]></constructor>
    </implementation>
  </binding>
</bindings>