The main thread on this topic has 14,000 views, but seems to have gotten buried and replaced by two more “Firefox doesn’t work” threads.
You can read about how we have been debugging this for a while on Launchpad.
I recently posted a new build cross-compiled for Ubuntu 16.04. Please try this out: https://www.dropbox.com/s/ogdut56m2i0k0t9/firefox_59.0.2%2Bbuild1-0ubuntu0.16.04.3_armhf.deb?dl=0 (download and dpkg --install). It seems to require quite a bit of RAM so on a Pi 2 or 3 you’ll likely get better performance by adding a gigabyte of swap space.