I used to have gtk2 in USE variable. Emerging mozilla it compiles right and installs without errors but the page about:plugins shows 0 (zero) plugins. No libnullplugin.so is there in /usr/lib/mozilla/plugins, and then if I install whichever plugin it doesnt work (for example flash or java).
azarah@nosferatu gentoo-x86 $ grep gtk2 profiles/use.desc gtk2 - use gtk+-2.0.0 over gtk+-1.2 in cases where a program supports both. beware that gtk+-2 support can be bad. azarah@nosferatu gentoo-x86 $ Mozilla is an example of this.