You can't. It's a different program and you can't access it's memory.
Maybe you can write a plugin/extension or you can monitor the network for out going requests.
But I really doubt that that can be done in Java.
"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." --- Martin Fowler
Please correct my English.
None. The operating system is in charge over memory management and will not allow you to read another programs memory. They go to great lengths to prevent this. The language of the plugin depends on the browser (I guess). What are you trying to accomplish? Because maybe we can suggest another solution.