You can install the extension on this link.
The extension is too young and it possibly has some bugs. The extension user interface is not very nice for a while but I hope people will find out how to use it.
It is an open source extension under MIT licence. The forum topic is not a support page, use github issue page to let me know if there are problems with the extension.
upd
upd2
All is fine, there is mistake in a mozilla documentation, the function name is wrong but the function itself works fine. Closed and open Shadow DOM is fully supported.
Since now I freeze the extension developing due to lack of a free time. I am going to fix some bugs if I have a time. And the same for all other my extensions.