I would take this question over to Mozilla.org and not here at askubuntu.com except for the following:
- This is a brand new install of Ubuntu, Firefox and Thunderbird.
- On the same computer I also have band new installs of Firefox and Thunderbird for the Wondows7 partition. And they are running perfectly.
- One of the Firefox lockups is totally reproducible. Simply put, I can right click on a link and Firefox will lock up every time.
It is almost like pop up menus (other then the Thunderbird password pop up) never show up for either application. In all lock up cases the application window darkens as if to draw attention to a pop up that never shows up. I am stating to suspect the pop ups actually do happen but are somehow not shown.
I should add, after the locked application window darkens, I can still (usually) click on the Ubuntu "X" button to ForceQuit the application. That force quit pop up does say "The window this-n-that is not responding".
Removing pulseaudio prevents the lockup.