In order to get started on Bug 1696573, we'll want to create a pref to "hide" the ongoing screenshots refactor. This will help us land patches and ensure we do not break current screenshots functionality. 1 will be in progress, and the other is the current implementation.
This is very similar to how we implemented the new print preview with
print.tab_modal.enabled . If the pref is true, taking a screenshot should bring up the new screenshots UI (maintained as a browser component). If false, we should operate with the current setup.
We'll have to add the pref to
StaticPrefList.yaml, similar to the printing pref .
Then, in the places where we usually make calls to the screenshots extension UI  , we should notify the observers if the pref is false. As a reference, here is how the printing service would accomplish that: https://searchfox.org/mozilla-central/rev/2c991232499e826e46f9d976eb653817340ba389/toolkit/components/printing/content/printUtils.js#38-43,342-343