I've been a user of portable versions of the major browsers for some time now. I could be wrong about this, but my strong impression is that if the installed Java is not physically present within their App "tree" (folders structure), and nothing has been explicitly set up in the browser to point to where your Java lives, the browser remains unaware of it. (This may or may not apply to the Microsoft browsers -- I don't know.)
Besides being able to run them off of a stick -- wherever you happen to be -- one of the things that appeals to me about the portable apps is that they typically do not involve the Registry, and the OS is barely aware of their existence, if at all. This should allow you to have more than one version of a given program on call, if you wanted to, and to avoid clashes between them.