Comment 2 for bug 494053

I can confirm this.

When Firefox enters this "state", quitting it still leaves the process on the background (at least for several minutes), triggering the "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system." when attempting to restart Firefox. After killing the process and restarting Firefox, sound works normally in Flash. I have seen the same symptoms on two different machines already.

Running:

Ubuntu 9.10 with 2.6.31-16-generic
Firefox 3.5.6
pulseaudio 0.9.19
Shockwave Flash 10.0 r42

I'm willing to provide additional information if needed, but I don't know what else to supply at this point.