Arch Linux

Description:
I'm unable to run firefox because the newest package version seems to lack a dynamic lib called libvpx.so.4
I could use firefox normally in previous version.

Additional info:
* package version: 48.0
* config and/or log files etc:

$ firefox
XPCOMGlueLoad error for file /usr/lib/firefox/libxul.so:
libvpx.so.4: cannot open shared object file: No such file or directory
Couldn't load XPCOM.

There's no libvpx.so.4 in packaged files:

$ pacman -Ql firefox | egrep "lib.*.so" ⏎
firefox /usr/lib/firefox/browser/components/libbrowsercomps.so
firefox /usr/lib/firefox/gmp-clearkey/0.1/libclearkey.so
firefox /usr/lib/firefox/gtk2/libmozgtk.so
firefox /usr/lib/firefox/liblgpllibs.so
firefox /usr/lib/firefox/libmozavcodec.so
firefox /usr/lib/firefox/libmozavutil.so
firefox /usr/lib/firefox/libmozgtk.so
firefox /usr/lib/firefox/libxul.so


Steps to reproduce:
- Update from firefox 47.0.1 to firefox 48.0
- Execute firefox command on terminal.
- Error message is shown.