Comment # 3 on bug 1204074 from Wolfgang Rosenauer

Firefox always selects the "latest" support libavcodec.
If you have libavcodec59 it will be used. Firefox has no idea that the
installed library does not support all codecs.
The solution would be to install the "correct" libavcodec59 package (I've got
the one from packman).

So this is either a bad dependency in Chromium or no bug (local repo setup).

Firefox recommends libavcodec-full >= 0.10.16 to fetch a useful libavcodec
package if available.