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
in the address bar and choose Disable Protection on This Page from the dropdown menu.
To allow insecure content to be displayed in all secure pages, enter about:config in the address bar and double-click on this preference, to toggle it from true to false:
You can also use this add-on instead, to toggle the preference:
- Toggle Mixed active content add-on
You may want to do this only for the current session, then reset the preference back to "true" and check back with the site to see whether it's been fixed, since this is a global setting that exposes you on all sites, not just the one you care about.
Let us know if this helps!