Firefox's clientHeight and scrollHeight

I am experiencing problem with detecting an overflow inside a div. I'm using firebug to debug my html.

In my firebug I see that my div has the same clientHeight and scrollHeight, even though Firefox rendered it with a vertical scrollbar (which should mean the scrollHe开发者_JAVA百科ight > clientHeight right?)

Is this a known bug? I haven't tested it with another browser. Are there any workaround for this?

FYI my div is overflow:auto and I'm using Firefox 6.0