Are you asking about a Java Security prompt?

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Java security prompts

Even after allowing Java for a site, you may still see a Java dialog box, asking you to confirm that you want the Java content to run. These security alerts are from Java, not Firefox. For more information, see the Java help page What should I do when I see a security prompt from Java?.

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Thank you!

P.S. I found the following with a google search:

http://answers.yahoo.com/question/index?qid=20140117012605AAyJ4hb Yahoo, you must know about applications that are being blocked by the latest version of Java (V7, Update 51)? Quote:
When you get a window that says "Your security settings have blocked an untrusted application from running" because of the latest Java software, you can work around the problem through the Java Control Panel by adding to the Exception Site the URL that is being blocked.

Another Yahoo Help thread suggests setting the Java Security slider from High to Medium (seems risky to me ... I would try adding an Exception URL first): http://answers.yahoo.com/question/index?qid=20140115090411AA6vsOD KI-GAME140115: "Application Blocked be Security Settings" on game launch?

If this describes your problem, then this is an issue with Yahoo Games and Java, not Firefox. Starting in Java 7 Update 51, sites that would normally be blocked based on your Java Security settings can be added to an Exceptions list, via the Java Control Panel. See the Java Help page How can I configure the Exception Site List? for details. If this doesn't work you'll need to contact Yahoo Help.

Update:

Java now has a web page that specifically mentions adding Yahoo Games to the Exception Site list:

http://www.java.com/en/download/help/java_blocked.xml Why are Java applications blocked by your security settings with the latest Java?