Everything is working as expected.
Go to
https://midimusic.github.io/browsertest/index.html with the old ffmpeg-libs-2.8.2 (0c00) and under diagnostic audio, click the mp3 link, music plays. Update to 4.2.2-1.0c00, close Firefox or SeaMonkey in my case and try again, no music, click the ogg link and music. We need a ffmpeg-legacy-libs package or such.
@Remy, the RPM version of theorae1.dll is missing that import while my build has it, both have it in theora0.dll not sure what is the difference in the build process that created the difference but yes, mixing one of my FFmpeg builds with the new RPM theora will crash. Workaround with LIBPATHSTRICT and BEGINLIBPATH for now. Not sure why your crash shows AVCODE57 as Firefox shouldn't try to load that one. Needs more investigation.
@Dariusz, the FFmpeg libs are dynamically loaded by Firefox, so pstat may not show it, libavcodec has a dependency on libavutil, you're seeing their 8.3 name versions.
Are you sure that Firefox pulled in AVCODE57? The build shouldn't matter, I think even FF 45.5 will show the same behaviour.
@David, did you enable netlabs-exp?
Everyone, there is probably problems with other versions of FFmpegs libraries on your LIBPATHS.
One thing is that it is easy to downgrade the libs, remove the main FFmpeg package if it is still there, I didn't have it installed and in ANPM, right click on the ffmpeg-libs package and choose "Get specific release" and pick 2.8.6-2