Here is a screenshot of the issue from jansmartist that she sent to me.

Thanks to both of you. Yes... this affected quite a few Mac users when they updated to Firefox 56.

Firefox 56 ratcheted up the security sandbox, limiting which files pages can use on your system. This apparently prevents Firefox from using font files in a "suitcase" or "Adobe Type 1" format. The developers are aware of this and considering solutions for a future version of Firefox.

If you can fix this by copying or moving common TrueType or OpenType format fonts into your system folder, like Arial, that might be best. However, as a Windows person, I don't know how you actually do that.

As a temporary workaround in Firefox, you can reduce the sandbox security level back to what it was in Firefox 52-55 as follows:

(1) In a new tab, type or paste about:config in the address bar and press Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste sand and pause while the list is filtered

(3) Double-click the security.sandbox.content.level preference and change the value from 3 to 1 and click OK

That change won't take effect until the next time you quit and restart Firefox.

Does that let Firefox show website fonts normally?

Hopefully this workaround is only necessary for Firefox 56 as this should be fixed in Firefox 57, so remember to reset the pref via the right-click context menu to the default value once you have updated to Firefox 57 in a few weeks since this is a security feature.