If you are using Windows you might want to give Pale Moon a try, on my system it is much smoother than the official Firefox/Aurora/Nightly.

Pale Moon feels as smooth as the official Firefox does under OS X, for whatever reason the official Firefox for Windows feels terrible on my system (clean profile of course).

Just out of interest, where have you found these performance issues?

The only thing I've noticed was that it really struggled to scroll down large pages whilst playing a video in another window/tab. Although that quickly fixed itself when I updated my graphics card drivers (completely forgot to do so after installing Ubuntu).

Sometimes just opening a new tab.

Facebook seems to lag my whole pc, that might be facebook's fault, but it doesn't happen in chrome.

I browse reddit quite a lot, images just seem to render slower (and the render all distorted until finished).

Youtube videos seem to go out of focus constantly.

Cold start is quite slow, though not unacceptable times, definitely slower.

When I load a big page, it seems to take much longer before I can start scrolling.

None of this are actually tested, so some may be perception, but in general terms, it actually feels quite slower, not bad, but not snappy.