Some Browser don't always work well with outlook. I would say use IE or Edge since that is the preferred for outlook emails.

Thanks but I would really like to continue to use firefox especially since I use a Mac. It worked before but at some point stopped working.

Hi michael2711, are you still using the older Outlook Mail, or did you switch to the Beta?

When testing the older Outlook Mail, I see the same thing. A window pops up, but there's no visible content.

This problem doesn't exist in Beta, but it's hard to launch a message into a separate window in beta -- instead of double-clicking, you need to use a menu on the message (screenshot attached).