Redcore

I don't think it could be firebug - it merely manipulates the view within the browser, not the actual file.

It's a bizarre problem if it's not check-out. I've never run into this problem, and I've used DW every version since DW8. How are you accessing the files? Are they on your system? All I can think of is that maybe you have a connection to the server set somehow and it's syncing up while you're trying to save.