It looks fine for me on Windows 10.
I went ahead and waited patiently for Firefox 47 to come out, because
this is still an issue on my machine. I know I said it was fixed, but
after more use, it was back to the same again and I just thought I would
wait till 47 came out, so I uninstalled firefox on my Mac, and
installed ONLY Firebug 3.0 and Firefox 47, but the issue was still
there. I made sure I deleted my Firefox profile on the filesystem as
well. I don't know why it works well in Nightly, but regular Firefox,
with no profile, the layout is still broken. You can see the screenshots
attached. UPDATE: I also fired up my old Mac Mini, which did not have Firefox on it, and installed Firefox 47 and Firebug 3.0, and Firebug is still having a layout issue on Mac only.