I wish I were not correct. I'd like to be more wrong.
Without knowing the ins and outs I too am disappointed that Mozilla orphaned old Thimble stuff. Well you can get to it, somewhere I was able to get to the old thimble to edit my stuff, though when I look now on their new site, it says my projects should be available but sadly they are not listed as promised under "my projects"
Oh wait here they are
it says they are migrating old projects over tomorrow, hah.
But I found your source, there is a URL in the page you tried for the iframe
grab that HTML while you can!
And congrats on the new position, Brian told me a little about it.