Same thing happened to me. The fix below worked. The other thing you can do is switch to 'Basic" mail in settings and then download and then switch back to enhanced when done.
Starting in Firefox 23, if a secure page includes certain types of insecure content, Firefox blocks the insecure content and a shield icon will appear in the address bar. See this article for more information about the new mixed content blocker: How does content that isn't secure affect my safety?
To temporarily allow the mixed content to be displayed:
Click the shield icon Mixed Content Shield in the address bar and choose Disable Protection on This Page from the dropdown menu.