Ctrl-O: Open File dialogue opens up
DON'T OPEN A FILE!
Now, go to your Gmail, find a Word attachment, click on Download, then select "Open."
That's all you gotta do; it will start hanging up right away, and eventually will lock up completely, and--here's the kicker--if you close Firefox you can't re-open it because it thinks it's already running. You have to restart your machine.