www/firefox: fix dependency version information
According to:
build/moz.configure/nspr.configure (NSPR, sets minimum version) build/moz.configure/nss.configure (NSS, sets minimum version) gfx/harfbuzz/README.mozilla (harfbuzz embedded copy, we use our own) gfx/graphite2/README.mozilla (graphite2 embedded copy, we use our own) media/libpng/README (libpng embedded copy, we use our own) media/libvpx/config/vpx_version.h (libvpx embedded copy, we use our own)
may this help me when I touch it next time
(cherry picked from commit 31c1a83b5c002170ca78ef9e6e0e51864dc1ce89)