When Firefox uses bitmap fonts, it can happen that on certain web pages the fonts are very ugly (compared to Google Chrome for example). To fix that, just disable bitmap fonts for X:
# ln -s /etc/fonts/conf.avail/70-no-bitmaps.conf /etc/fonts/conf.d/
found here … https://wiki.archlinux.org/index.php/firefox#Firefox_uses_ugly_fonts_on_certain_web_pages