Every so often Firefox will get a tiny bit confused about where the keyboard focus should be - on the address bar, in a field in a form, or on the web page itself. I've noticed this happen and it gives me behavior like you describe - the copy command isn't actually available because the selection and the focus aren't held by the same widget.
I've never had to restart, though. Just a few clicks here and there to get the two realigned fixes it for me.
I've never had to restart, though. Just a few clicks here and there to get the two realigned fixes it for me.