In case you missed it Mozilla finally released an official 64bit version.
http://arstechnica.com/information-technology/2015/12/firefox-for-windows-finally-has-an-official-stable-64-bit-build/
I had to go back to the download page and download the 64bit edition but it upgraded right over top and all my firefox native extentions seem to work fine.
Not sure about external plugins however.
Feels very quick.
I think I'm going to wait a little bit before upgrading again, not that I run a lot of plugins... just wary I guess