The link to https/thimble.webmaker.org starts out as an 'untrusted connection' because the SSL certificate was issued to tbpl.mozilla.org... rookie mistake guys. Alwats test your links first ...
Anyways, I had a quick look at the site, not that great. I was thinking it would be some kind of trainer program that you installed on top of your browser that would hold your hand. But it turns out to be a collection of user created projects with just one that has all the "training" parts in comments inside the code.... not so good to train people with...
Mozilla is going back to their roots. Remember when the Netscape browser had built-in web page creator? Then it got stripped down and became the popular Mozilla which then became the even more popular Firefox. Fortunately it is now separate from the browser though.