I see most of the Unicode characters on Firefox/Gentoo Linux with the following packages installed:

    media-fonts/arphicfonts
    media-fonts/baekmuk-fonts
    media-fonts/cardo
    media-fonts/corefonts
    media-fonts/dejavu
    media-fonts/droid
    media-fonts/font-bh-lucidatypewriter-100dpi
    media-fonts/font-bh-lucidatypewriter-75dpi
    media-fonts/font-bh-ttf
    media-fonts/font-bh-type1
    media-fonts/freefont
    media-fonts/freefonts
    media-fonts/inconsolata
    media-fonts/intlfonts
    media-fonts/kochi-substitute
    media-fonts/symbola
    media-fonts/terminus-font
    media-fonts/ttf-bitstream-vera