User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

browser.pagethumbnails.capturing_disabled = true

Edit a "Top Site", add a Custom Image URL, Preview

Actual results:

"Image failed to load. Try a different URL."
Same issue reported here https://support.mozilla.org/en-US/questions/1231726

Expected results:

The image should be used, same as if browser.pagethumbnails.capturing_disabled = false.

This also happens in Nightly 68 2019-05-12 Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Here's the browser console error

getScreenshot(https://www.mozilla.org/media/img/logos/firefox/logo-quantum.9c5e96634f92.png) failed: TypeError: NetworkError when attempting to fetch resource. Screenshots.jsm:45
    getScreenshotForURL resource://activity-stream/lib/Screenshots.jsm:45
    throw self-hosted:1227

The problem might be going through mozilla-central/source/toolkit/components/thumbnails/BackgroundPageThumbs.jsm#102 function captureIfMissing

This is a hidden pref used for testing purposes only. The user does not have any about:preferences UI to edit this preference so I don't consider this a valid bug report.

Depends on bug 1674252.