Ok. I set it up when I was not paying attention so I kinda didn't keep track of what it was I did. I reconstructed the steps the best I could. YMMV:
This was for firefox 1.0 on a redhat 9 box. I assume it is very similar for other distros.
0. download http://www.monachos.net/other/fonts/SPIONIC_.TTF
1. exit all running copies of firefox
2. as root, mkdir /usr/share/fonts/TTF
3. as root, chmod 755 /usr/share/fonts/TTF
4. as root, mv SPIONIC_.TTF /usr/share/fonts/TTF/SPIONIC.TTF <-- notice no '_'
5. as root, chmod 644 /usr/share/fonts/TTF/SPIONIC.TTF
6. edit $HOME/.mozilla/firefox/<xxx>.default/prefs.js and add this to the bottom:
user_pref ("font.FreeType2.enable", true);
user_pref ("font.freetype2.shared-library", "libfreetype.so.6");
// if libfreetype was built without hinting compiled in
// it is best to leave hinting off
user_pref ("font.FreeType2.autohinted", false);
user_pref ("font.FreeType2.unhinted", true);
// below a certian pixel size anti-aliased fonts produce poor results
user_pref ("font.antialias.min", 0);
user_pref ("font.embedded_bitmaps.max", 1000000);
user_pref ("font.scale.tt_bitmap.dark_text.min", 0);
user_pref ("font.scale.tt_bitmap.dark_text.gain", "0.0");
// setup font dirs for TrueType fonts
user_pref ("font.directory.truetype.1", "/usr/share/fonts/ttf-bitstream-vera");
user_pref ("font.directory.truetype.2", "/usr/share/fonts/TTF");
user_pref ("font.directory.truetype.3", "/usr/share/fonts/corefonts");
user_pref ("font.directory.truetype.4", "/usr/share/fonts/freetype");
user_pref ("font.FreeType2.printing", true);
7. stop editing the prefs.js file (don't have it open/in a buffer/etc.)
8. restart firefox
9. visit Textkit and stare in awe at the correct fonts.
10. I assume that one you do this, many other TrueType fonts can be added at will simply by sticking them into /usr/share/fonts/TTF and selecting the correct fonts in the HTML.
I performed the above steps on my install to test it, and it seemed to work. You should probably find one person to test the steps on who doesn't have it working before you make it public. It would suck to give out wrong information that just generates inquisitional traffic. If it is wrong somehow, I'll figure it out and give you the right answer.