Ok, eu substituí 'Droid' por 'Droid Sans', 'Droid Serif', etc., e funcionou:
vic@wic:~/Documents$ fc-match sans
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
DroidSans.ttf: "Droid Sans" "Regular"
vic@wic:~/Documents$ fc-match serif
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
DroidSerif-Regular.ttf: "Droid Serif" "Regular"
vic@wic:~/Documents$ fc-match sans-serif
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
DroidSans.ttf: "Droid Sans" "Regular"
vic@wic:~/Documents$ fc-match mono
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.
LiberationMono-Regular.ttf: "Liberation Mono" "Regular"
vic@wic:~/Documents$
Em seguida, adicionei mais padrões ao arquivo e agora as fontes são substituídas no navegador:
<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<alias>
<family>serif</family>
<prefer>
<family>Droid Serif</family>
</prefer>
</alias>
<alias>
<family>sans-serif</family>
<prefer>
<family>Droid Sans</family>
</prefer>
</alias>
<alias>
<family>sans</family>
<prefer>
<family>Droid Sans</family>
</prefer>
</alias>
<alias>
<family>monospace</family>
<prefer>
<family>Liberation Mono</family>
</prefer>
</alias>
<!-- Replace Arial, Times New Roman, Verdana and Courier New -->
<!-- with their counterparts. Strong binding used to override -->
<!-- distribution defaults -->
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Arial</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Dorid Sans</string>
</edit>
</match>
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Times New Roman</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Droid Serif</string>
</edit>
</match>
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Verdana</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Droid Sans</string>
</edit>
</match>
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Helvetica</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Droid Sans</string>
</edit>
</match>
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Courier New</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Liberation Mono</string>
</edit>
</match>
<match target="pattern" name="family">
<test name="family" qual="any">
<string>Courier</string>
</test>
<edit name="family" mode="assign" binding="strong">
<string>Liberation Mono</string>
</edit>
</match>
<match target="font">
<edit mode="assign" name="rgba">
<const>none</const>
</edit>
</match>
<match target="font">
<edit mode="assign" name="hinting">
<bool>true</bool>
</edit>
</match>
<match target="font">
<edit mode="assign" name="hintstyle">
<const>hintmedium</const>
</edit>
</match>
<dir>~/.fonts</dir>
<match target="font">
<edit mode="assign" name="antialias">
<bool>true</bool>
</edit>
</match>
</fontconfig>