I'm not familiar with Shift+Ctrl+p being a "hotkey" or keyboard shortcut for another function. Was it specific to Firefox and possibly a feature of an add-on, or is it something on your system that works in multiple applications?

Are you sure you remember the correct hotkey?

Ctrl+Shift+P has always been the hotkey for New Private Window or switching on PB mode. Code for Firefox 10 esr:

• http://mxr.mozilla.org/mozilla-esr10/search?string=key_privatebrowsing&find=&findi=&filter=^[^\0]*%24&hitlimit=&tree=mozilla-esr10

Was that function copying data to the clipboard?