The biggest problem at that time was that the mouse didn't work on the menus. I ran the project again today and I guess my JDK is different (newer) than what I was using at that time because now that problem is fixed, without any changes in my code, i.e., something has been enhanced on the JavaFX level.
Click to enlarge the image below to see a JavaFX application on top of mFX:
The remaining problem is that the window system is still in Swing. If the tabs could somehow be rewritten to JavaFX (and I have tried several things without success) then the entire application would be stylable via CSS.
Aside from that, it's looking pretty good. Above, you see a CSS-styled label that updates itself, via NetBeans Lookup, based on whatever the user has chosen in the selector window in the top right. There's an editor module, a viewer module, and a domain module, in other words, mFX truly enables you to do modular JavaFX development, using all the techniques provided out of the box by the NetBeans Platform, i.e., modularity, loose coupling, and pluggability.