Firefox 7.2, USE=webm

[wcg]

Should the Firefox 7.x ebuild perhaps check to see if sound is
disabled (USE="-alsa -oss") before enabling the USE=webm
use flag and pulling in an emerge of libvpx, alsa-headers,
and alsa-lib?

(I have a system where I install Xorg, xfce4, and firefox for
an emergency gui console, but it does not have sound. It
was a little susprising to see alsa-headers and alsa-lib in
the "emerge -DuNp system" list with "USE=-alsa -oss" in
/etc/make.conf.)

edit: "emerge -DuNp world", not "emerge -DuNp system";
Firefox is in the world set, not the system set.
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TIA