Firefox limits NPAPI support (technology required for Java applets)

64-bit Firefox

The 64-bit version of Firefox does not support NPAPI plug-ins, including Java.

Firefox 52 and above

Beginning with Firefox 52 (released March 2017), plug-in support is limited to Adobe Flash, and drops support for NPAPI, impacting plugins for Java, Silverlight, and other similar NPAPI based plugins.

If you have problems accessing Java applications using Firefox, Oracle recommends using Internet Explorer (Windows) or Safari (Mac OS X) instead. Developers and System administrators looking for alternative ways to support Firefox users should see this blog regarding Launching Web Start applications.

Firefox 52 ESR 32-bit release

Mozilla offers an Extended Support Release (ESR) version of Firefox specifically for use by organizations who need extended support for mass deployments. Only Mozilla Firefox 52 ESR 32-bit release will continue offering support for the standards-based plugin support technology required to launch Java Applets. To see if you are using an ESR release, check the Firefox menu item (Help -> About) and looking for the "ESR" identifier.

Mozilla maintains Firefox ESR Releases for approximately one year. Developers and users still relying on the Java plugin technology in the 32-bit Mozilla Firefox web browser should consider migrating to a different solution.

Related

» Firefox plugin support (Java Product Management blog)
» Plugin support and Firefox 52 (mozilla.org)
» NPAPI Plugins in Firefox (mozilla.org)

Firefox 42 crashes when trying to run the Java plug-in

NOTE: The workaround applies to Firefox 42, and requires Windows Java Version 8 Update 66 Build 18 or later versions. » See JDK-8133523 for related information.

Workaround

A few options are listed. The simplest method is noted first and recommended. Other available options are not recommended for non-technical users.

Option 1 (Simplest method)
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
   Add Preference name as dom.ipc.plugins.java.enabled.
6. 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);

Mozilla references

» http://kb.mozillazine.org/User.js_file
» http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

Firefox warning message: Add-ons may be causing problems

Cause

Older versions of the Java Deployment Toolkit (DT) and the Java Plugin have been disabled by Firefox. Some Java versions are flagged by Firefox as 'Add-ons may be causing problems'.

If Firefox thinks an add-on is unsafe and can cause a security risk, it will block it from loading. In case of a soft block Firefox will present a warning message but will allow the user to continue if they wish but with compromised security. In the case of a hard block Firefox will block installation so that the user cannot use this add-on. To resolve this, the user should update to latest version of Java.

Workaround

Upgrading to the latest Java version should resolve the issue. If you see the warning message, follow these steps:

1. Ensure that the Disable box is checked.
2. Click on Restart Firefox.
3. Download the latest version of Java..
   

Java not working in Firefox due to stand-alone JavaFX installation

Cause

This could be due to a conflict that Java versions 7u10 and above have with stand alone installations of JavaFX. Example: If Java 7u5 and JavaFX 2.1.1 are installed and if Java is updated, Firefox is unable to detect that the newly installed Java.

Workaround

Remove stand-alone version of JavaFX (Windows)

1. Find and remove all versions of JavaFX and Java through the Windows Uninstall Control Panel.
   
2. Run the Microsoft utility to repair corrupted registry keys that prevents programs from being completely uninstalled or blocking new installations and updates.
3. Reinstall Java

• Java (mozillaZine) (mozilla.org)
• Use the Java plug-in to view interactive content on websites (mozilla.org)