Firefox ugly font size with Xfce4

Check your resolution

~$ xdpyinfo |grep resolution
  resolution:    75x75 dots per inch

Open 
~$ emacs ~/.config/xfce4/Xft.xrdb&

Xft.antialias: 1
Xft.hinting: 1
Xft.hintstyle: hintfull
Xft.rgba: rgb
Xft.dpi: 76

Restart gdm or whatever..

One useful command to check your Xft configuration

xrdb -q |grep Xft

OR,

in the firefox
about:config
Set the value of layout.css.dpi to 0 , which means that Use the host system’s logical resolution for interpreting dimensions specified in absolute units.

more detailed information is the following....