Firefox: Add ctrl+option+left/right to jump tabs

autoconfig.js

// Note this goes in /usr/lib/firefox/defaults/pref/autoconfig.js
pref("general.config.filename", "firefox.js");
pref("general.config.obscure_value", 0);
pref("general.config.sandbox_enabled", false);

firefox.js

// IMPORTANT: Start your code on the 2nd line, Note this goes in /usr/lib/firefox/firefox.js
try {
  let wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
           .getService(Components.interfaces.nsIWindowMediator);

  // Global state to track AltGraph
  let isAltGraphDown = false;

  function handleKeyEvent(window, event) {
    // Update AltGraph state
    if (event.key === "AltGraph") {
      isAltGraphDown = event.type === "keydown";
      return;
    }

    // Check for our shortcuts
    if (event.type === "keydown" && event.ctrlKey && (event.altKey || isAltGraphDown)) {
      if (event.key === "ArrowLeft") {
        window.gBrowser.tabContainer.advanceSelectedTab(-1, true);
        event.preventDefault();
      }
      if (event.key === "ArrowRight") {
        window.gBrowser.tabContainer.advanceSelectedTab(1, true);
        event.preventDefault();
      }
    }
  }

  let windowListener = {
    onOpenWindow: function(xulWindow) {
      let window = xulWindow.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
                           .getInterface(Components.interfaces.nsIDOMWindowInternal ||
                                      Components.interfaces.nsIDOMWindow);

      window.addEventListener("load", function() {
        if (window.document.documentElement.getAttribute("windowtype") === "navigator:browser") {
          window.document.addEventListener("keydown", e => handleKeyEvent(window, e));
          window.document.addEventListener("keyup", e => handleKeyEvent(window, e));
        }
      });
    },
    onCloseWindow: function() {},
    onWindowTitleChange: function() {}
  };

  // Add listener for new windows
  wm.addListener(windowListener);

  // Handle existing windows
  let windows = wm.getEnumerator("navigator:browser");
  while (windows.hasMoreElements()) {
    let window = windows.getNext();
    window.document.addEventListener("keydown", e => handleKeyEvent(window, e));
    window.document.addEventListener("keyup", e => handleKeyEvent(window, e));
  }
} catch(ex) {
  Components.utils.reportError(ex);
}