Earlier quoted context omitted.
It was never that slow as some people declare it to be.
Yeah, it positively was , and it would lock the browser up with misbehaving tabs. I managed to use Fx50 (or so, I forget the exact release) for about a day before going "nope, back to Chrome, it might chew batteries like crazy but at least it's fast." Firefox has significantly improved and deserves praise for it. Rewriting history to always be at war with Eastchromea is unnecessary.
The benchmarks have always been respectable. I'm sure there are some configurations that made it work poorly, and that's a problem. That isn't true in general.