Firefox – Why does the sound stop working sometimes when watching youtube videos

Here is the situation:

While watching a youtube video, sometimes, the sound will stop working (even the beggining of the video, which I saw WITH sound, no longer plays the sound) but the image still works
(video keeps playing) Simply restarting firefox solves the issue..

update:
same problem occurs in epiphany (another web browser available for ubuntu) Restarting epiphany also solves the problem..

fyi: I use Firefox 3.5 on Ubuntu 9.10 Karmic koala

If there is anything more you need to help me out, I'll edit it in this post.

Thanks in advance!