Firefox has an update which impacts external pages. This feature can prevent users from seeing external pages. The details revolve around a security measure called 'Mixed Content Blocking.' This feature is by default enabled for all sites as of Firefox 23.
This feature blocks all HTTP (insecure) content if the main URL is HTTPS (secure). The biggest impact is on external pages that are not HTTPS pages. Those external pages will display as blanks. The Mozilla informational page describing this change, along with instructions on disabling it for specific sites is: