After years of compiling firefox I decided that I get no advantage from doing it :p
So I just emerged firefox-bin and discovered that it doesn't find any of the plugins in /usr/lib/nsbrowser/plugins. The gentoo ebuild patches the source code to use nsbrowser/plugins for the default plugins location, but the firefox-bin ebuild can't patch the source code so it does something different, and on this machine it doesn't work. I just finished downgrading this particular machine from ~x86 to plain x86, so there may indeed be something wrong in my general environment. Anyone here running firefox-bin, and how does it find your plugins? (There are at least two easy ways to fix the problem. You can make a symlink from /usr/lib/nsbrowser to /usr/lib/mozilla, or you can make a symlink from /opt/firefox/plugins to /usr/lib/nsbrowser/plugins, either method works.)