Firefox Fonts – How to Override Website Fonts in Firefox

Edit: I completely updated this answer after getting some breakthrough advice from a colleague.

Here's what I inserted in /etc/fonts/local.conf (inside the <fontconfig> element):

<!-- Replace Helvetica with Arial -->
<match target="pattern">
    <test qual="any" name="family">
        <string>Helvetica</string>
    </test>
    <edit name="family" mode="assign" binding="strong">
        <string>Arial</string>
    </edit>
</match>    

Similarly for Times -> Times New Roman. (See my full local.conf here.) The key was to use binding="strong" for the <edit> element. (Also, using "assign_replace" mode instead of "assign" causes something similar, except that then it's too aggressive: also Verdana gets replaced with Arial).

Changes in font configurations are effective immediately. Besides testing in Firefox, you can check that it works like this:

$ fc-match helvetica
Arial.ttf: "Arial" "Normal"

If you run into problems, the best help is near: man fonts-conf. (Although even with the documentation, the workings of the font system seemed somewhat complicated or unwieldy to me.) You can also try to "debug" what's really going on using a command like:

FC_DEBUG=4 fc-match helvetica

In addition, FC_DEBUG=1024 fc-match helvetica shows the list of config files that affect the font matching.

Boy oh boy. So the answer lies in reinstalling Arial.ttf. Apparently when I installed the fonts from corefonts.sf.net in an attempt to fix the issue it didn't actually install the font but placed it in the %windir%\fonts directory. It wasn't until I accessed the font and then clicked "Install" at the top of the preview windows that it was installed. Shouldn't have trusted that executable installer. Now both IE9 and FF4 are displaying correctly.