How to enable copy/paste in Mozilla/Firefox
 
An easier way to do it is this:

1. Open a new Tab with CTRL+T, and browse to 'about:config'.
2. Right click in the main area, select New->String.
3. Enter 'capability.policy.policynames'
4. Enter 'allowclipboard'

Repeat steps 2, 3 and 4 using the following pairs of values:

capability.policy.allowclipboard.sites  -  http://www.yoursitedomainname.com
capability.policy.allowclipboard.Clipboard.cutcopy  -  allAccess
capability.policy.allowclipboard.Clipboard.paste  -  allAccess

Finally, exit the browser.

The 'about:config' window will not be updated to show your additions, but your pref.js file will be updated when you exit the browser. You can confirm this by browsing to your profile directory as indicated above,and opening the prefs.js file.