But even then, in the case of Firefox, only the respective hostname and the port will be trusted. The Accept the risk and proceed option will not be available for Applications Manager.
Here are a few other ways that might help you remediate this problem:
Upload a CA certificate to OpManager so that the browser can trust both OpManager and the Applications Manager plug-in. (Highly recommended)
Access Applications Manager as a standalone product from the browser, and then accept the risk, if you want to continue. Now, access the OpManager UI. Click here to know about the steps to find the hostname and port number of the APM plugin.
Under Firefox, go to "Settings -> Privacy & Settings -> View Certificates -> Servers -> Add Exception", and then add an exception for the Applications Manager URL in the Firefox Certificate Manager.