I am on Windows 7 64-bit using Firefox 7 and JDK 7. Firefox is not able to detect the Java plugin and I am not able to run java plugins.
Perhaps you installed the 64-bit Java plugin but since Firefox is likely 32-bit, you actually need the 32-bit Java plugin instead?
Did you check that the Java plugin is installed on firefox ?
EDIT : you can see here
If Java is not working, make sure that the Java plugin is enabled in the Add-ons window Manager tab:
At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP), and then click Add-onsOn the menu bar, click on the Tools menu, and then click Add-onsAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will openAt the top of the Firefox window, click on the Tools menu, and select Add-onsOn the menu bar, click on the Tools menu, and select Add-onsAt the top of the Firefox window, click on the Tools menu, and select Add-ons. The Add-ons window will open.
In the Add-ons Manager tabwindow, select the Plugins panel.
Click on the Java (TM) Platform pluginJava Embedding Plug-InJava Plug-in 2 for NPAPI Browsers (Mac OS 10.5 & 10.6) or Java Applet Plug-in (Mac OS 10.7)Java plugin to select it.
Click on the Enable button (if the button says Disable, Java is already enabled).
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.