This functionality is completely removed since 2013-12-14 18:23 PST, see: bugs 38966 and 913734

Note: If you've reached this page from a message box in Firefox or another Mozilla product, try using keyboard shortcuts for the Cut, Copy, and Paste commands:

The information on the rest of this page is for Web developers and advanced users. Please do not try to modify this page.

To protect users' private information, unprivileged scripts cannot invoke the Cut, Copy, and Paste commands in Midas, which is Mozilla's rich text editor component. This means that the corresponding buttons on the Mozilla Rich Text Editing demo page will not work. To enable these functions, you must modify your browser preferences.

Warning: Changing these preferences can leave your browser insecure, especially if you grant permission to untrusted sites. Only change these settings as needed to try the demo above and to test your own add-on or Firefox-internal code, and be sure to restore the default settings when you're done!

Changing the preferences in Firefox

  1. Quit Firefox. If you have Quick Launch running (on Windows, this is an icon in the toolbar), quit that too.
  2. Find your Firefox profile directory.
  3. Open the user.js file from that directory in a text editor. If there's no user.js file, create one.
  4. Add these lines to user.js: 
    user_pref("capability.policy.policynames", "allowclipboard");
user_pref("capability.policy.allowclipboard.sites", "https://www-archive.mozilla.org");
user_pref("capability.policy.allowclipboard.Clipboard.cutcopy", "allAccess");
user_pref("capability.policy.allowclipboard.Clipboard.paste", "allAccess");
  5. Change the URL https://www.mozilla.org to the site for which you want to enable this function.
  6. Save the file and restart Firefox. The Clipboard buttons in the demo, or similar buttons on the sites you listed, should now function.
Note: The preference is site as well as protocol specific. For example: 
user_pref("capability.policy.allowclipboard.sites", "http://www-archive.mozilla.org")

is not the same as:

user_pref("capability.policy.allowclipboard.sites", "https://www-archive.mozilla.org")

This is because the first uses HTTP while the second uses HTTPS.

If you want to allow multiple URLs to access the Paste operation, separate the URLs with a space. For example: 
user_pref("capability.policy.allowclipboard.sites", "https://www-archive.mozilla.org https://developer.mozilla.org")

Again, keep in mind the security risks involved here and be sure to remove permission to access the clipboard once you no longer need it enabled.

See also