On non-Firefox but NPAPI-capable browsers such as Safari, Chrome and Opera, there is currently no way to interact with the browser's cookie, proxy and authenticator support because there are no entry points defined in the NPAPI to access this functionality. Currently even the new Java Plug-In uses XPCOM APIs private to Firefox for this purpose.
Thanks to collaboration with Mozilla, new NPAPI entry points have just been added to be able to get and set cookies, query the browser's proxy settings, and look up authentication information. These entry points will be available starting in Firefox 3.1 and it is expected they will quickly be ported to other web browsers. The Java Plug-In needs to be updated to take advantage of them once they are available.