Binary package hint: compiz
The other day, I had firefox running and my home directory is mounted on a md raid drive. That day the md drives were syncing and the file read time was incredibly long. I happened to be right clicking on firefox when firefox stopped responding. Compiz made it turn gray, but after it did that, I couldn't click on anything again. The menu that firefox poped up was not gray only firefox was gray. The display hadnt' crashed or anything else. The applications in the background were running smooth. Then after a minute, firefox came back and everything was back to normal until the next time firefox had to access files from the HD. I believe it happened with the right click menu and the toolbar menu on top.
I think compiz is waiting for the program to accept the closure of the menu, but the program is not responding. The menu has a special functionality that when it is open, all of the clicks outside of it are only to close the menu and cant be used for anything else. Thus in this wait the user cannot interact with her desktop at all. I was lucky enough that firefox had not actually crashed. I am not sure what would have happened if it actually did. (I was planning on logging into my computer from my laptop to kill firefox, but it came back before I had the chance.)
Thanks for taking the time to report this bug and helping to make Ubuntu better. Could you please add your ~/.xsession-errors, /var/log/Xorg.0.log and the output of lspci -vvnn to your bug report as separate attachments? Thanks in advance.