I don't remember specifically what they were anymore, but I do remember back when that happened there were still 2-3 features in Firebug not in the official tools that I used regularly. I think one was around ajax requests, the other around CSS. I ended up holding off on the Quantum upgrade for about a year for them.
There is also Firefox developer edition. Its dev tools are a bit more comprehensive than those of vanilla Firefox. I wonder how it compares to Firebug's feature set.
