When Firebug breakpoints into a web page, that web page's JS is halted,
but of course Firebug's JS in the XUL window continues to run. So there
are (at least) two "JS threads" (I don't know if they are related to OS
treads).
But somehow Firebug has more than one or what I don't know. I am hoping
someone can shed some light on the issues.
jjb