1. When we deploy to production, our fabfile runs `commonplace includes`. 2. This writes to a file called `build_id.txt` a numeric timestamp. 3. Zamboni uses this timestamp to cachebust our favicons, stylesheets, and scripts (which are served from our CDN and have a `Cache-Control` `max-age` of 10 years). See view-source:https://marketplace.firefox.com/ 4. If a resource hasn't changed, we still refetch the resource because the URL is different. There are times (seldomly, albeit) when our stylesheets or scripts haven't changed. The favicons and splash screen change almost never. The main stylesheet and main JS bundle - those are almost guaranteed to have changed every push. Anyway, we shouldn't be blindly cachebusting things if the content hasn't changed, like this one for example: https://marketplace.cdn.mozilla.net/media/fireplace/img/logos/128.png?b=1395166222469