Linux – Firefox 57 cannot display Arial on Linux

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.

This specific problem is due to installing a version of the Helvetica font (very commonly used on the web) that either Windows or Firefox can't display. To confirm, all you have to do is paste the following text in your Firefox address bar and hit enter:

data:text/html,
Helvetica
   <font face="Helvetica" size="25">abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ</font>

Helvetica Neue
  <font face="Helvetica Neue" size="25">abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ</font>

If it displays as weird symbols, then you can be sure that's your problem.

To fix it, all you have to do is go to %WINDIR%\Fonts and delete all the fonts named Helvetica.

If for some reason, Windows doesn't allow you to, you have to go to the registry, navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts and then delete all the entries that have Helvetica on their name.

If you are using Windows 7 64-bit, you also have to delete the Helvetica entries from the following registry key.

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Fonts

After this, you only need to reboot your computer and the fonts will be displayed correctly again. At least that fixed it for me.