Web fonts not displaying correctly in some browsers

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.

I've seen that happen before to a workmate. Removing the font and re-installing solved the issue. If you need assistance removing fonts from OSX and reinstalling follow this link

EDIT:

To clear the current user’s font cache, first quit all running applications (otherwise, you may see font oddities after using the command), then open Terminal (in Applications -> Utilities) and run this command:

atsutil databases -removeUser

If you want to remove the font cache for all users, use this command, and provide your admin password when asked:

sudo atsutil databases -remove

Once you’ve cleared the caches, you should stop and restart the ATS server with these commands:

$ atsutil server -shutdown

$ atsutil server -ping

It will take a second or two for the server to restart, but when it does, you should be good to go. If anything still looks odd, a logout and login should take care of any lingering issues.

credit: http://www.macworld.com/article/1139383/fontcacheclear.html