FireFox SDK keydown / keyup events

Is it possible to track keydown/keyup events in FireFox addons? I would like to implement something similar as following scenario:

After pressing and holding the modifier key panel appears
While holding these modifier keys, pressing the other keys will cause some actions with the panel
When modifier key released, the panel disappears

Solution

You would need to register your event listener in each browser window for that. The high-level SDK API don't give you direct access to the browser window however, you will have to use the low-level modules for that, in particular the (currently undocumented) sdk/keyboard/observer module. It allows you to listen to key events in all browser windows, so something like this should work:

// Register key event handlers in each browser window
var {observer} = require("sdk/keyboard/observer");

observer.on("keydown", function(event) {
  // Ignore events that have been handled elsewhere (e.g. by the web page)
  if (event.defaultPrevented)
    return;

  if (...)
    panel.show();
});
observer.on("keyup", function(event) {
  // Ignore events that have been handled elsewhere (e.g. by the web page)
  if (event.defaultPrevented)
    return;

  if (...)
    panel.hide();
});

Notes:

sdk/keyboard/observer module is completely undocumented, it might change or go away completely any time.
Originally the solution proposed here used WindowTracker from the sdk/window-utils module which is now deprecated. If you really want to look at browser windows yourself you would now use the (also undocumented) sdk/windows/observer module which allows listening to open and close events. The function isBrowser() to distinguish browser windows is now available via sdk/window/utils module. You would also need to use windows() function to consider already open windows, the windows observer doesn't do that automatically.