Double-click on the Firefox icon to launch Firefox. If it is already running, click File and then Close before starting the application again.
At the top of the screen, click on the Tools option. Select Add-ons from the list of options. Click on Plugins located on the left side of the screen. A list of already installed plugins appears on the screen.
From the listed plugins, find the one that corresponds to Java. If you have a Windows computer, this is labeled Java Platform. On a Mac computer, it is labeled Java Applet Plug-in.
Click on the Java drop-down menu to reveal the Java browser options. Choose one of the first two options, which include Ask to Activate or Always Activate. If you select the Ask to Activate option, you must enable Java each time it is activated within the browser. To do this, click the Allow Now button when asked whether to allow Java to run.