I just installed Firefox 17 ESR. Before I was on 18 (latest). I noticed that the hack I put in my profile prefs.js and user.js to allow clipboard paste,cut and copy operations no longer works. I tried clipboard helper add-on it is not working either. So, I created a new profile, put the following text in user.js and prefs.js
Hmmm, even if you are permitting clipboard access through the capability mechanism, starting in Firefox 17, sites can no longer use the old privilege elevation request system. I believe they need to upgrade the scripts (by signing them with a digital signature) in order to manipulate the clipboard.
Is this critical for your application? In other words, can you get by with the standard Windows methods for cut/copy/paste:
Ctrl+x / Ctrl+c / Ctrl+v