1. Since Firefox 3.6, the Java is treated just like all other plugins and no longer has a enable check box in Tools > Options > Content. We can enable or disable Java plugin (and also all plugins) via Tools > Add-ons > Plugins.
2. What if the Java plugin does not appear in Tools > Add-ons > Plugins (you can also checked it by typing about:plugins in the address bar). It means that you have to install Java (before you can use Java in your Firefox). Since Firefox 3.6 and later versions you need the Next-Generation Java™ Plug-In present in Java 6 U10 and later.
These are how you can do it in various operating system (taken from http://kb.mozillazine.org/Java):
On Mac OS X
You can use the Mac OS X Software Update feature to update Java.
Note: The Java Embedding Plugin is bundled with current Mac distributions of Mozilla browsers, including Firefox and SeaMonkey. This utility allows browsers other than Apple's Safari to use the most recent versions of Java on Mac OS X.
On Linux
1. Download the Linux version of the Java software.
2. When the download is finished, execute the .bin file as root and follow the prompts.
3. When the installation has completed, run this command in the plugins directory of your Firefox installation:
ln -s /your_path_here/java/j2re1.5.0/plugin/i386/ns7/libjavaplugin_oji.so libjavaplugin_oji.so
4. Firefox 3.6 and later need the Next-Generation Java Plug-In
See Manual Installation and Registration of Java Plug-in for Linux (at java.sun.com)
Run this command in the plugins directory of your Firefox installation to create a symbolic link to the Java plugin (replace xx with the current Java version):
ln -s /usr/your_path_here/java/jre1.6.0_xx/lib/i386/libnpjp2.so libnpjp2.so
On Solaris Intel
1. Install the Solaris Intel version of the Java JRE or SDK.
2. When the installation has completed, run this command in the plugins directory of your Firefox installation:
ln -s /your_path_here/jre/plugin/i386/ns610/libjavaplugin_oji.so .
On Windows
Automatic update: The Java Update feature automatically checks for updates in Windows 2000/XP/Vista at scheduled intervals and notifies you when an update is available so that you can install it. If you have administrative privileges, you can also check for updates yourself and install them from the Java Control Panel (Start -> Control Panel -> Java -> "Update" tab).
Manual install or update:
1. Go to Java.com and click on the Free Java Download button.
2. Click on the Download Now button to start the online installation.
3. Close and restart your browser after the installation is complete.
If you prefer an offline installation, go to the Java SE Downloads page (see above for links) select the Java SE Runtime Environment (JRE) and save the offline installer (e.g., "jre-6u10-windows-i586-p.exe" for JRE 6 Update 10) to any convenient location, close your browser and run the installer.
Important: Previous to JRE 6 Update 10, installing or updating to a newer Java version would add a new Java Runtime Environment (JRE) instead of replacing the existing version. For example, if you already have JRE 6 Update 7 installed and later install JRE 6 Update 10, you would end up with two JRE versions installed in separate directories. You can remove older Java versions via Windows Control Panel "Add or Remove Programs", to save disk space and because older versions often contain security vulnerabilities.
Starting with JRE 6 Update 10, future Java updates will patch the current version by default instead of adding an additional version. For example, if you previously installed JRE 6 Update 10 in the C:\Program Files\Java\jre6 directory, and in the future you install JRE 6 Update 14, the version 6u14 installer does not create a new directory. Instead, it updates the pre-existing "jre6" directory with the new 6u14 content (JRE Update 10 will no longer exist). This will prevent an accumulation of unused and potentially insecure older JRE versions.