Bug#1127569: firefox-esr: should support/recommend libavcodec62 (for libvpx consistency, and libavcodec61 is no longer built)

[Stephane Chazelas]

https://bugzilla.mozilla.org/show_bug.cgi?id=1962139 has "wontfix" 
against the firefox-esr140 branch, so I suppose Debian's only option for 
forky is to provide libavcodec61 at least until the next esr major 
version.