Firefox 4 strange dialogs

[Jim Byrnes]

Using Firefox 4.0.1 on Ubuntu 10.04. I'm seeing dialog boxes that have 
no title bar and no icon next to the text displayed.  All I see is the 
border of the dialog with a grey background, the text message and any 
OK,Cancel etc buttons.  This seems to happen with modal dialogs but 
non-modal ones seem to display as expected.

An example would be the dialog Yahoo mail pops up when you delete the 
last message from the spam folder.

Is anyone else seeing this and know of a fix?

Regards,  Jim