How to set breakpoints using firebug

I'm using Firebug (for the first time) on Firefox 3.6.8 on MacOS X 10.6.7. The Firebug documentation says that you can set a breakpoint just by clicking on the source line. That's not happening for me -- it's like the source window doesn't have focus or somethin开发者_如何学Pythong.

Here's the browser screen duump (below). What do I need to do to set breakpoints and debug a simple local script?

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Go to the Script tag on top, then click a line number there.