If you want to try something else, there is also the Pale Moon
project, they produce FF builds specifically optimized for Windows, both in 32- and 64-bit. The current version is 9.1.
At the same time the current Nightly (version 13 as unbelievable as it sounds) is rather stable and I can't even reproduce the problem reported in this thread. I have no problem watching YouTube or anything else for that matter. Of course, I have very few plugins and extensions. Flash is the official Adobe 64-bit, Java is version 7 64-bit. The Office plug-in is also 64-bit ( I use 64-bit Office), although I am not sure what it does really. There are a couple of things such as Password exporter that I needed when I replaced my motherboard recently, which I used successfully despite the Mozilla claims of incompatibility, but now I have all of that disabled. That's it, no problems with this at all.