Comment 3 for bug 347376

I'm confirming the trouble is with adobe's flash plugin.
Firefox's window where any flash element is active like video or sound but not banners.
Firefox doesn't crash and the window can still be properly closed.
Same problem under opera but as they are wrapping the plugin it's okay by closing the tab.

I've recently disabled pulseaudio trying to fix glitchy sound so it may be the cause.