I thought this got solved to some extent when recent builds of Firefox started working on 18.04 Bionic. However, the lack of an official release of Ubuntu MATE 18.04 for Pi has limited the benefit for much of the casual community.
Recently I needed to do some performance testing so I reran the cross-compiling flow I had used for Firefox 59.0.2. You can download this build of Firefox 64.0 for Raspbian Stretch here: https://github.com/jdonald/firefox-armhf/releases I have also updated the GitHub repo with notes adapted for codebase changes over the past year.
For some reason it was picking the wrong armv6 instructions with WebRTC so I had to build with --disable-webrtc. I can probably find a fix for this (ideal for Pi webcam streamers), but in the meantime looking to hear how this build runs in general. For a best experience, don't forget to run sudo raspi-config and explicitly enable the full OpenGL driver with KMS.