javascript engine and thread safety

Well, I was doing some simple AJAX at work and was wondering how will the engine handle script safety.

I remember when I was debugging the javascript engine for zoloFighter, the engine appears to be single thread only.

Some search on the internet, I found this : http://odetocode.com/Blogs/scott/archive/2007/01/08/9734.aspx
The author seems to agree with me but still it is not clear.

So it is time to try it out! Sure enough, both IE and Firefox serialize everything on one thread only :-)