Firefox prevents clipboard control on default, and this is for security reasons. Picture a user always using the clipboard to copy/paste his bank password, or something like that, ANY website run on Internet Explorer can "grab" his clipboard contents, and save it, leaving the user clueless to what just happend, and he'll never know better.
That's one of the best reasons of using Firefox. More secured than IE.
There is a way to disable the clipboard disabled features, and even run a list of allowed sites.
Here is the script for Firefox:
//To allow paste function to work in firefox you must:
//write in url about:config
//and change signed.applets.codebase_principal_support = true
window.clipboard = new Clipboard();
alert("If you are using firefox please do the following :\n 1. Write in your url box : 'about:config'\n2. Change signed.applets.codebase_principal_support = true\n")
// Must init the clipboard
window.onload = init;
// Function to return the data in clipboard