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

This article applies to:
  • Browser(s) Firefox
  • Java version(s): 7.0, 8.0

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.


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 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
    Add Preference name as
  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("", 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("", false);
Mozilla references


Firefox warning message: Add-ons may be causing problems

The following message appears while launching Firefox browser with the warning:
Firefox has determined that the following add-ons are known to cause stability or security problems


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.


Upgrading to the latest Java 7 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

Upon completing install of Java 7u10 and above, following error appears:
Error: A plug-in is needed to display this content. Install Plugin


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.

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 uninstall utility to repair corrupted registry keys that prevents programs from being completely uninstalled or blocking new installations and updates.
  3. Reinstall Java


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