There are whitelisted websites that you can access during mochitests using support-files, for example: http://example.com/browser/browser/extensions/activity-stream/test/functional/mochitest/blue_page.html This would allow setting that url as a topsite in mochitests and actually clicking and navigating to it without triggering a network requests. Currently setting a default topsite without having an image defined in top_sites.json would trigger a screenshot request which fails because tippytop sites don't have a sharedCache method [0] Is it possible to return early if link.__sharedCache is not present or should we look for a different solution? [0] https://github.com/mozilla/activity-stream/blob/master/system-addon/lib/Screenshots.jsm#L101

If we have the decision to not make screenshots for default sites, then it should probably be done from TopSitesFeed.jsm instead of Screenshots.jsm. For our packaged defaults, we'll probably make sure there's an associated packaged tippy top icon, so the decision is more for other situations: testing, user-set custom defaults, distributions, etc. One would probably expect screenshots or whatever equivalent behavior from actually having the page as a frecent top site, so disabling screenshots probably isn't the right approach.

> One would probably expect screenshots or whatever equivalent behavior from actually having the page as a frecent top site Agreed. I suggested preventing screenshots because that is the current behavior (more or less). Maybe the purpose of the bug should be handing this failure > JavaScript error: resource://activity-stream/lib/Screenshots.jsm, line 102: TypeError: cache is undefined When editing the default topsites pref.