OK, if you have doublechecked everything and still have that same problem I would suggest the following:

- install x-ttcidfont-conf package
- install msttcorefonts package
- install freefont package
- install ttf-bitstream-vera package
- install ttf-xfree86-nonfree package

That should get you a fine collection of truetype fonts to start. Now, go edit your /usr/X11R6/lib/X11/fs/config (deliberately using that long path to ascertain file is in place) and make sure you have /var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType/ path listed in the catalogue line. Even better, copy this (it's proven to work) and put it in the config file:

catalogue = /usr/share/fonts/X11/misc/,
/usr/share/fonts/X11/100dpi/:unscaled,
/usr/share/fonts/X11/75dpi/:unscaled,
/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType/,
/usr/share/fonts/X11/Type1/,
/usr/share/fonts/X11/100dpi/,
/usr/share/fonts/X11/75dpi/

It should all be on one line (broken here just for presentation purposes) and without spaces between paths. Restart xfs, restart firefox. Now, do you see any difference, do you see text in flash now?