Client cannot see invoice in Firefox

The client cannot see the preview of their invoice in Firefox. Have checked/unchecked the “Live Preview” and “Prevent Download” checkboxes in Account Management to no avail. Any suggestions?

Maybe this will help?

https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

Also - ask them to try disabling their ad-blocker on your billing site.