Apparently you are right.
And if the clipboard has not been used by SP after the boot, the r/c menu in FF works flawlessly.
Is it your opinion that the misbehavior is caused by Firefox? Like the OpenOffice issue is caused by OOo, the EditPlus issue by EditPlus, the Notepadx64 issue by Notepadx64, the 25Clip issue by 25Clip?
I must admit, I am wondering...