Java isn't a browser extension. There is a Java runtime that can be used a browser plug-in, but I don't think this is the same Java required by Multibit.
not to interrupt the discussion on how to best serve the blockchain off ramdisk (not being sarcastic, very pleased if the thread has some actual utility) but can anyone clarify or expand on the above statement?
Java has different types of executable programs:
1. Just an application (ran via command line on a local computer) [Multibit]
2. Servlets (ran on a server inside a web server)
3. Applets (ran in a web browser like Flash) [browser plug-in]