Put this in your ~.fonts.conf, and set Arial in your XFCE control panel.
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<match target="font">
<test name="family" qual="any">
<string>Arial</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test name="family" qual="any">
<string>Helvetica</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test name="family" qual="any">
<string>Helvetica Neue</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test name="family" qual="any">
<string>Courier</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test name="family" qual="any">
<string>Courier New</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test name="family" qual="any">
<string>Tahoma</string>
</test>
<edit mode="assign" name="antialias">
<bool>false</bool>
</edit>
</match>
<match target="font">
<test compare="more" name="weight">
<const>medium</const>
</test>
<edit mode="assign" name="antialias">
<bool>true</bool>
</edit>
</match>
<match target="font">
<test compare="more_eq" name="size" qual="any">
<double>14</double>
</test>
<edit mode="assign" name="antialias">
<bool>true</bool>
</edit>
</match>
<!-- Firefox -->
<match target="font">
<test compare="more_eq" name="pixelsize" qual="any">
<double>15</double>
</test>
<edit mode="assign" name="antialias">
<bool>true</bool>
</edit>
</match>
<match target="pattern">
<test name="family" qual="any">
<string>Helvetica</string>
</test>
<edit binding="same" mode="assign" name="family">
<string>Arial</string>
</edit>
</match>
<match target="font">
<test compare="eq" name="family" qual="any">
<string>Skype UI Symbol</string>
</test>
<edit mode="assign" name="antialias">
<bool>true</bool>
</edit>
</match>
</fontconfig>