The trick is the `link` argument - sometimes it's there and sometimes it's not. I got lost trying to track down exactly when `data-direct-link` is added. Just by throwing some alerts in `trigger_ie_download`, it seems the download button on the home page does the `window.open` bit, while the button on /firefox/new/ does not.

I dug into the download_buttons.py helper, but ran out of steam around here: https://github.com/mozilla/bedrock/blob/master/bedrock/mozorg/helpers/download_buttons.py#L219