This should be the counterpart of Java Class Library. Java Class Library does not support FX; but is can not currently be replaced by an instance of "library" FX Application due to the fact that FX Application build infrastructure is optimized to create launchable deployment artifacts.
This type of project is really needed as there are cases when you want to create parts of application that doesn't have any run method but depend on jfrt.jar. Adding it manually doesn't seem to be a nice solution as it goes into project's lib folder and require additional work to maintain correct link.
I'm not sure why issue 166531 was closed and would like to investigate the problem in more detail.
From NetBeans 7.4 onwards this is no longer an issue, because standard Java Class Library now takes over the role of the originally intended "JavaFX Class Library". This is made possible by changes described in Issue #229705.
To use standard Java Class Library for JavaFX development, go to Project Properties, Deployment panel, and check "Keep JavaFX Runtime on Classpath". The underlying update mechanism will keep the correct jfxrt.jar and possibly other FX RT artifacts on classpath, even after changes of current Java Platform.