That depends on what you call a "stable" firmware.
It's tagged as a "beta"-release, but it works just fine, I'm using it since it was released by defector.
As far as I understand, 1.21 is based on 1.19 but with some of the more advanced features removed - possibly an attempt not to overburden the "normal user", but that's simply my personal speculation.
I don't remember all the differences, but one huge feature that only exists in 1.19 is Karma style playlisting. This is the ability to independently create and edit saved playlists without having to load them as running playlists first.
I've had loaded firmware when it came out in 2008(!). I don't know the reasons why, but I was disappointed of that version and went back to 1.19. In my opinion it's not worth to install.
I agree Interneci. The anounced features are improvements to trekstor's first release and do not contain all the great stuff coded by the defector crew.
The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" but "That's funny..." (Isaac Asimov)