If you are having trouble with FireFox, close ClipMate completely with File | Exit ClipMate.
Then copy something from FireFox, and paste into Notepad.
If that does not work, then FireFox is broken, or more likely, you have some malware, likely known as AdWare.Win32.AdvertMen.a
Please see these articles:
This article is dedicated to my friend Winsleuth, (Dave Hungerford of Vero Beach), who dug up most of this information.
Or, you may be having trouble with another exploit, such as the Adobe Flash clipboard attack which hijacks your clipboard.