<flag name="gmp-autoupdate">Allow Gecko Media Plugins (binary blobs) to be automatically downloaded and kept up-to-date in user profiles</flag> However enabling of "gmp-autoupdate" USE flag results in setting media.gmp-gmpopenh264.autoupdate=false in configuration.
You could use usex: for plugin in gmp-gmpopenh264 ; do echo "pref(\"media.${plugin}.autoupdate\", $(usex gmp-autoupdate true false));" >> ...
the problem with usex is that in the USEflag-enabled case I don't want to set any default pref -- that is, I want the pref to be empty/unset instead of being set to true. Anyhow, i already fixed this in-tree: + 04 Dec 2014; Ian Stakenvicius (_AxS_) <axs@gentoo.org> + +firefox-34.0.5-r1.ebuild, -firefox-34.0.5.ebuild: + fixed inverted logic on gmp-autoupdate flag, bug 531624 +