I found the following, give it a try and see if it helps you.
The Firefox copy/paste failure has been traced to a piece of junkware called New.net (NewDotNet) which is being bundled out with a lot of genuine
applications at the moment, and installs itself in Firefox as if it were an extension.
Symptoms include the copy/paste buttons being grayed out on the context menu and the clipbord being cleared when attempting to copy between tabs or Firefox windows.
To remove new.net you must use the Add/Remove programs feature in Control Panel, or using its own uninstaller if it has been kind enough to leave one lying around, or via
Uninstalling NewDotNet
http://www.newdotnet.com/removal.html or by downloading and running MS Antispyware and removing anything it recommends to do with new.net
Do not bother attempting to remove via Firefox, it does no good.
There is a comprehensive report on New.net on this site:
http://www.cexx.org/newnet.htm
If you don't have NewDotNet, then use the Firefox Standard Diagnostic:
http://kb.mozillazine.org/Standard_diagnostic_(Firefox)