To test whether Java is installed and enabled in Firefox, visit one of these Java test pages:

   Verify Java Version
   Verify Java Version (alternate) 

When you visit these pages, you will normally need to activate Java. The article How to allow Java on trusted sites explains how. You may also see a Java security prompt asking you to confirm that you want to run Java. This security message is from Java, not Firefox. For more information see the Java help page What should I do when I see a security prompt from Java?. Enabling Java

If Java is not working, make sure that the Java plugin is enabled in the Add-ons Manager tab:

   Click the menu button New Fx Menu and choose Add-ons. The Add-ons Manager tab will open.
   In the Add-ons Manager tab, select the Plugins panel.
   Click on the Java(TM) Platform plugin to select it.
   If Java is disabled, click on Never Activate to open its drop-down menu and select the option to activate Java. The recommended (default) option is Ask to Activate, which lets you run Java when prompted. 

Note: You only need to enable the "Java(TM) Platform" plugin, if it is disabled. The "Java Deployment Toolkit" plugin is used by Java developers to detect your Java version and to deploy their Java applications and does not need to be enabled for Java to work. Troubleshooting

If Java is installed but doesn't work, try these solutions: Java plugin does not appear in the Add-ons Manager

If the Java plugin does not appear in the Add-ons Manager, even though Java is installed, try each of the following, one at a time, and then reopen Firefox to see if that resolves the problem: Remove the pluginreg.dat file

With Firefox completely closed, delete (or rename) the pluginreg.dat file, located in the Firefox profile folder (it will be regenerated once you restart Firefox but any other plugins you may have disabled will be enabled). See Re-initializing the plugins database for detailed instructions. Make sure Java is enabled in the Java Control Panel

Starting in Java 7, the Java Control Panel includes the Security option, "Enable Java content in the browser", which is selected by default. De-selecting this option will prevent Java applets from running in any browser and the Java plugin will no longer appear in the Firefox Add-ons Manager. Make sure that Enable Java content in the browser is selected in the Java Control Panel Security tab. See this Java Help page for details. Make sure the 32-bit version of Java is installed

On 64-bit Windows, if you only have the 64-bit version of Java installed, the Java plugin will not be available. Current Firefox releases are 32-bit browsers and you will need to install the 32-bit version of Java. Make sure plugin scanning is not disabled

Mozilla browsers use plugin scanning to locate installed plugins so you should make sure that the plugin.scan.plid.all preference has not been disabled:

   In the Location bar, type about:config and press Enter.
       The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise! to continue to the about:config page. 
   Search for the preference plugin.scan.plid.all
   If it's been set to false, right-click on it and select "Reset". 

Uninstall JavaFX if installed

In some cases, after updating from an earlier Java version to Java 7 Update 10 or above, the Java(TM) Platform plugin is no longer detected. This can be caused by a conflict with Java and a standalone installation of JavaFX that prevents the Java plugin from being properly registered. To resolve the problem, uninstall JavaFX. For more information, see this java.com help page. Other solutions

If Java is installed and enabled (see above) but isn't working properly, try the following: Remove outdated Java plugin files

Make sure that multiple versions of the Java(TM) Platform plugin are not being detected. For example, if Java 7 Update 51 is currently installed, the Java(TM) Platform plugin in the Add-ons Manager tab should be listed as Java(TM) Platform SE 7 U51. If you see other "Java(TM) Platform" versions listed, uninstall the older versions (see How do I uninstall Java on my Windows computer?) or else remove them manually (see Manually uninstalling a plugin). Check for other software that may block Java

Even if Java is installed and enabled in Firefox, it may be blocked by other software or Firefox add-ons, for example:

   The ZoneAlarm Pro software firewall can block Java. For more information on enabling Java through this program, see the ZoneAlarm Pro forum. 

   The NoScript extension for Firefox will block Java. For more information, see the NoScript FAQ. 

Additional resources

   The Java FAQs page at java.com answers some frequently asked questions. 

Based on information from Java (mozillaZine KB)

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Java 8 was just released; could that be the problem?

http://www.oracle.com/technetwork/java/javase/overview/index.html

The design of the plugin check site (last time I checked) doesn't accommodate multiple "current" versions. So if the site has been updated to recognize Java 8 as current, this could lead to a lot of confusion.

Assuming the Plugins section of the Add-ons page has not disabled Platform SE 7 U51 (based on the block file that Firefox regularly downloads), then I think it's probably still good.

(I actually have U45 on this computer, whoops, so I can't test the response to U51 right now.)

Sourjobraato Banerjee, please link to the article instead of copy+pasting almost all of the Windows page of https://support.mozilla.org/en-US/kb/use-java-plugin-to-view-interactive-content

Many thanks. Problem was that Java did not install V8 properly.