bootstrap.js

function startup() {
  let styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"]
                          .getService(Components.interfaces.nsIStyleSheetService);
  let styleSheetURI = Services.io.newURI("chrome://myaddon/skin/toolbar.css", null, null);
  styleSheetService.loadAndRegisterSheet(styleSheetURI, styleSheetService.USER_SHEET);
  // returns true
  console.log(styleSheetService.sheetRegistered(styleSheetURI, styleSheetService.USER_SHEET));
}

chrome.manifest

skin myaddon classic/1.0 skin/

skin_toolbar.css

@-moz-document url("chrome://browser/content/browser.xul") {
  #loop-button[cui-areatype="menu-panel"],
  toolbarpaletteitem[place="palette"] > #my-button {
    list-style-image: url(chrome://myaddon/skin/menuPanel.png);
    -moz-image-region: rect(0, 32px, 32px, 0);
  }
}