Create any simple application and make it to run as an Applet. Same execution as desktop or JNLP works fine.
The probleme has been introduced in dtfx.js in build 382 of soma.

Work fine (except for Firefox on Mac snow leopard):
http://jfx.sfbay.sun.com/hudson/job/soma/label=macosx-universal-soma/381/artifact/artifacts/deployed/dtfx.js

Is not working:
http://jfx.sfbay.sun.com/hudson/job/soma/label=macosx-universal-soma/382/artifact/artifacts/deployed/dtfx.js

Previewing in the Authoring Tool (and netbeans BTW) is not possible.