Looks like this post got missed. I can clearly reproduce this bug using Firefox 42.0 + Firebug 2.0.13. You may want to
create a bug report for it. Though note that the upcoming Firebug 3 is recreated from scratch and now based on the built-in DevTools, so it's functionality will be based on those tools.
Note that the Firefox DevTools have a similar issue, for which I now filed
bug 1227746.
Sebastian