I clicked the Screenshots button and it said that using Screenshots meant I accepted the terms of service. I wanted to know the features before reading the terms, so I clicked next. Somehow it considered that I had accepted the terms, and the popup closed. So I lost the link to the terms of service. The link needs to be included somewhere.
It's on the bottom of each screenshots page. Clicking the "My Shots" button, loading any existing shot, or going directly to https://screenshots.firefox.com/shots will give the link in the footer.
Clicking the "My Shots" button or loading any existing shot means that I'm using Screenshots, and thus implies I accepted the terms of service. And https://screenshots.firefox.com/shots is not obvious. The extension should appear in about:addons and in the description there should be the link with the terms.