hi, you're likely seeing https://bugzilla.mozilla.org/show_bug.cgi?id=1460917 which is triggered by third-party font managers changing the location of fonts on the system on older versions of os x. as a workaround in the meantime you could either switch to [https://www.mozilla.org firefox 61beta] which should already contain a fix for this problem or try the following temporary configuration change until the next update comes out: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named '''security.sandbox.content.level'''. double-click it and change its value to 2 or 1 and restart firefox once.