Hi Bob,
Have you tried to test this issue in any other browser on Mac OS? If you can view the emails by using another browser normally, and can send/receive emails in the Firefox on Mac, it indicates that this is a Firefox related issue.
Could you try the following steps to troubleshoot this issue?
1. If the Firefox is not the latest version, please update it to the latest version.
2. Update the Plug-ins, such as flash player etc.
3. Start the Firefox in Safe Mode.
4. If possible, test this issue on another Mac/Firefox.
Thanks,
Bourne Zhang