Someone may find this useful so I am posting it.
My wife is Thai and I have been struggling with getting Thai fonts to render properly in Slackware, especially with Firefox since this is her source of news back home.
After some digging around the net with Google, I came across these fonts:
I followed the example install directions and the fonts are looking great now. Before they were almost unreadable. Another plus is now I can read all files that are in Thai.
# cd /usr/share/fonts/truetype
# tar xzf ~/Download/thai-ttf-0.4.15.tar.gz
# mv thai-ttf-0.4.15/etc/fonts/conf.avail/* /etc/fonts/conf.avail
# ln -s /etc/fonts/conf.avail/64-ttf-thai-tlwg.conf /etc/fonts/conf.d/
# ln -s /etc/fonts/conf.avail/89-ttf-thai-tlwg-synthetic.conf /etc/fonts/conf.d/
# rm -rf thai-ttf-0.4.15/etc
Hope someone else finds this post useful.