broken Rated 2 out of 5 stars

Unable to install it anymore, xpi download only possible by extracting url from html code.
It shows successfully installed but didn't show up

This review is for a previous version of the add-on (0.18.0).  This user has a previous review of this add-on.

Nice way of misusing reviews...

Everything works fine in a fresh Firefox/profile. I am not sure what you mean by "show up." You need to enter a CoAP URI into the address bar, e.g.:
coap://vs0.inf.ethz.ch/

Kind regards
Matthias