I've just installed Firefox onto my Mac and I'm finding that I can't eject the disk from my desktop. I've moved Firefox into the Applications and tried going to "File" and ejecting from there, but I get the message that "Firefox couldn't eject because Firefox was using it." I have also tried shutting down everything, replacing Firefox in the applications and clicking on the icon with a white circle and cross, but I get the message "A copy of Firefox is already open. Only one copy of Firefox can be open at a time." I'm basically stuck. Any help would be appreciated, please.