RHEL6.10 Firefox update 60.9.0 breaks video playing

After updating my RHEL6.10 64bit firefox from 60.8.0 to 60.9.0, some website video playing breaks. Broken on various news outlet websites & Facebook. Downgrading back to 60.8.0 lets videos play again (without modifying my firefox profile).

I have some mpeg packages from nux installed if that's relevant.

# rpm -qa '*mpeg*'
libmpeg3-utils-1.8-7.el6.nux.x86_64
ffmpeg-libs-2.6.8-3.el6.nux.x86_64
libmpeg2-0.5.1-8.el6.nux.x86_64
gstreamer-ffmpeg-0.10.12-1.6.el6.nux.x86_64
libmpeg3-1.8-7.el6.nux.x86_64

Ideas?