How do I remove Security exceptions from Firefox?
Click the "View Certificates" button to open the Certificate Manager. There are several different panels here; click Servers to show the server certificate exceptions. If you find the unwanted one here, select it and click the Delete...
Why is Firefox blocking a website?
You are probably using an extension or a plugin which blocks certain content such as images & videos or advertisements. It may also be possible that your plugin is not updated and therefore firefox shows a red mark and asks you again, if its safe to play.