When the new plugin is running on 1.4.2 its cookie handling support will not work, since it relies on APIs introduced in 5.0. A forward port of the 1.4.2 plugin's cookie-related code to the new plugin will be needed. This code will be used when the new plugin is running an applet on a 1.4.2 JRE.
While making the 1.4.2 support work in the new plug-in it was also discovered that the proxy support also will not work as it is based around the java.net.ProxySelector API which was introduced in 1.5. Whatever mechanism was used in 1.4.2 will need to be forward ported to make this work.
It was also determined that the DeployCacheHandler mechanism doesn't work on 1.4.2 since it relies on the java.net.ResponseCache which was introduced in 1.5. It is unclear whether this needs to be supported on 1.4.2; we believe this only handles caching of arbitrary data the user downloads via URLConnection manually, which is useful but not critical.