This doesn't occur when, running the same build of firefox on the same machine, I forward X to another display.
It smells then like an X server or video driver bug (Xorg 1.4.0.90, fglrx 8.58.2). However, since the problem doesn't occur with Firefox 3.0, so it's still possible this is a 3.5 bug.
If you decide to resolve this as INVALID, I'll report it on Ubuntu's bug tracker.
This doesn't occur when, running the same build of firefox on the same machine, I forward X to another display.
It smells then like an X server or video driver bug (Xorg 1.4.0.90, fglrx 8.58.2). However, since the problem doesn't occur with Firefox 3.0, so it's still possible this is a 3.5 bug.
If you decide to resolve this as INVALID, I'll report it on Ubuntu's bug tracker.