Could you get us a regression range for this?
The issue is reproducible on all Firefox versions (v4-v10). On Firefox 4 I used http://getfirebug.com/releases/firebug/1.7/firebug-1.7.0.xpi for testing since Firebug 1.9 is compatible only with Firefox 5 or higher.
OK, I see the problem too. It looks like users can work around by pressing Ctrl+R again. I notice that on step 9, the code is actually paused at one of the breakpoints, and if I continue and make sure the code isn't paused, then it works correctly. So it looks like there is some glitch, possibly a race, where the code isn't seen if paused at a breakpoint. I think it may be a Firebug bug rather than a JS engine bug, although I'm not sure.
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