In future versions of Firefox, the browser will display recommended browser extensions that are related to the site that a user is visiting. These extension will provide extra functionality to the site such as enhancing search functions, protecting a user's privacy, or performing some other behavior.
This new feature is called Contextual Feature Recommender, or CFR, and will recommend extensions that Mozilla feels would be beneficial to users of various sites. Currently this feature is only available in the Firefox Nightly builds, but is expected to be rolled out in Firefox 63.
When enabled, if a user visits a site that Mozilla has a recommended extension, Firefox will wait for the page to finish loading, wait an additional second, and then display a "Recommendation" button to the right of the URL in the address bar. When a user clicks on this button, a "Recommended extension" door hanger will drop down that provides a brief description of the extension and an option to install it.
If a user closes the door hanger or does not interact with it within 5 seconds, the hanger will close and the Recommendation button will change to a small extension symbol as shown below. This button can then be clicked to open the recommendation again.
While I have only been able to trigger the Amazon Assistant recommended extension, Techdows.com who reported on this feature, was able to also show recommended extensions for Facebook, Reddit, and YouTube.
When the about:config window opens, search for browser.newtabpage.activity-stream.asrouterExperimentEnabled and double-click on it to set it to true.
Then close Firefox Nightly and start it again. Now when you visit various sites, if there is a recommended extension, Firefox will recommend it. Please note that this may be a limited test, so you may need to refresh the site numerous times before a recommended extension is displayed.