Yeah, the site is templated client-side using Ampersand (though lmorchard keeps threatening to rewrite it in React as a weekend project). Should work fine with tracking disabled, but won't work at all without JS.
Oh wow, this is one of the simple designs which only needs HTML and CSS, no JS at all to work, I wonder why they chose to do it this way.
It looks simple, but it actually does a lot of communication behind the scenes via JS with an add-on installed in the browser. Also, someday, this whole web app may end up embedded in Firefox itself.