Try the instructions here:
http://www.mozilla.org/editor/midasd...rityprefs.html
It apparently can fix your problem.
To summarise:
Close down Firefox, do a search for prefs.js. It should be in a directory like C:\Documents and Settings\<account name>\Application Data\Mozilla\Firefox\Profiles\ and then in a random folder like erfHgd.default. If there's not such file there then you'll need to create it. If the file is there, right click and open with notepad (or if it's not open notepad anyway) and add these line of code:
Code:
user_pref("capability.policy.allowclipboard.Clipboard.cutcopy", "allAccess");
user_pref("capability.policy.allowclipboard.Clipboard.paste", "allAccess");
user_pref("capability.policy.allowclipboard.sites","http://www.cableforum.co.uk");
user_pref("capability.policy.policynames", "allowclipboard");
Save the file as prefs.js and reopen Firefox. Copy and paste should now work for this site.