From Mozilla x86-64
Firefox x64 version has to use 64-bit plug-in. Even if you install 32-bit plug-in, it cannot use it on 64-bit version. If you use Linux, you can use nspluginwrapper.
Java™ Platform, Standard Edition 6u12 Releases is ready for Windows x64 with plug-in support. It is available from http://www.oracle.com/technetwork/java/javase/downloads/index.html.
Also, older release for x64 doesn't include plugin. 64-bit plugin is from 6u12 release. Since x86 version is only x86 plugins, you have to install x64 JRE if you want to run Java Plugin on x64 Firefox.
Adobe released preview version of Flash player "Square". It is available from http://labs.adobe.com/technologies/flashplayer10/. Stable version release is inclued x64 plugin now. x64 plugin doesn't support sandbox.
Microsoft releases x64 plugin as Sliverlight 5.