>Honestly, I thought Firebug merged with Firefox a long time ago, since they both did what I needed, in almost identical ways.
Me too. I seem to recall that back when the Firefox inspector and friends first launched they stated that it would be able to do everything that Firebug did, and that the people who had been working on Firebug were going to join Mozilla to work on Firefox instead. I switched from Firebug to the builtin tools soon after it was released.
Speaking of the builtin tools, I found an old announcement about the Page Inspector 3D View [1] which I recently wondered what happened to since it disappeared not long after it was introduced. Apparently it's available as an addon instead now so you can still have it [2] [3].
[1]: https://blog.mozilla.org/blog/2012/03/13/firefox-adds-new-de...