We thought long and hard about including Firebug wholesale and considered several approaches to integrating it. An early prototype of the Inspector even include

We thought long and hard about including Firebug wholesale and considered several approaches to integrating it. An early prototype of the Inspector even included a significant portion of Firebug. Ultimately, integration proved to be too challenging and would have required rewrites that would have been equivalent to starting over. Source: https://hacks.mozilla.org/2013/10/firefox-developer-tools-and-firebug/ https://hacks.mozilla.org/2013/10/firefox-developer-tools-an...