I had Firefox 45 beta 8 running, and all I needed to do was fetch libc and libcx from Netlabs-exp, and install them with RPM. Then I rebooted.
I unzipped (p7zip) the distribution and made objects based on the new location of firefox. I did do the yum update specificed in the readme, and it fails in ways that are predictable since I put in newer libc and libcx.
It works. Printing isn't going to make people very happy, because only postscript printers are supported. My maps bug is apparently fixed, or at least better than it was.