So the libvpx transition prompted me to take a look at this, I added some code to debian/rules to create a fake homedir, use it for the build and remove it in the clean target. Unfortunately I then ran into another failure.
/firefox-esr/media/webrtc/trunk/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc:858:17: error: âstruct vpx_svc_ref_frame_configâ has no member named âframe_flagsâ sf_conf.frame_flags[layer_idx] = layer_flags;
I have no idea what to make of this. My google searches aren't turning up anything useful.