Java plug-in does not work in Firefox after installing Java

Tested on Firefox 42 (old release) and Sea-Monkey 2.46/2.49.

There are several workaround options. The simplest method is noted first and is recommended. The remaining available options are not recommended for non-technical users.

Option 1: Add property name dom.ipc.plugins.java.enabled in Firefox preferences

1. Launch Firefox
2. Type about:config in the address bar
3. You will be presented with a Firefox warning about the configuration settings. After acknowledging the Firefox alert, you should see a listing of the configuration preferences.
4. Right-click anywhere inside the displayed list of preferences. Select New then select Boolean.
5. Add property name as dom.ipc.plugins.java.enabled
6. Add Preference name as dom.ipc.plugins.java.enabled.
7. A window will be displayed, select false. Then select OK. 

Option 2: Create “user.js” file with preference entry

Instructions are based on Mozilla references listed below.

1. Use a text editor to create a “user.js” file in your Firefox profiles folder
2. Add entry: user_pref(“dom.ipc.plugins.java.enabled”, false);  

Option 3: Update “pref.js” file with preference entry  

1. Locate the “pref.js” file in the Mozilla profiles folder
2. Find and update preference: user_pref(“dom.ipc.plugins.java.enabled”, false);

Src: https://www.oracle.com/technetwork/java/javase/8u66-relnotes-2692847.html