Jan Beich reported that --enable-av1 doesn't work with --with-system-libvpx, at least on x86. This is because media/libaom/moz.build uses VPX_ASFLAGS and VPX_AS_CONVERSION which are not set when the build is configured to link to the system copy of libvpx. VPX_AS_CONVERSION isn't so difficult to replicate, but there are a number of conditions on VPX_ASFLAGS, currently set in old-configure.in. Probably the best thing it to port that whole section to moz.configure and make it available to both libvpx and libaom moz.build files.
In the real world, once this gets to the point where it can make a release one would think that anyone building with "--with-system-libvpx" would also build with the yet to be added "--with-system-libaom".