Java Development Kit has been permanently disabled for security reasons.

Gmail doesn't need the Java Development Kit, it doesn't even need Java.

Note Java Plugin and JavaScript in Firefox are two different things. JavaScript was originally going to be called LiveScript which would have kept some from confusing the two over the years.

http://kb.mozillazine.org/JavaScript_is_not_Java

When you posted, the forum polled your plugins that have either "Ask to Activate" or "Always Activate" permission, and found the Java Deployment plugin. To enable "Ask to Activate" plugins on a website, you can:

Look for the plugin notification icon in the address bar -- it typically looks like a small, dark gray Lego block. When the page wants to use a blocked plugin, the icon turns red to alert you to the concern. Click that and you can trust the site to use the plugin.

However, as noted earlier, Google's site does not use this plugin, so if a page on Google's site seems to want you to activate this plugin, your Firefox or your system might be hacked.