There seem to be some issues affecting the recommendations shown in about:addons, while in Private Browsing Mode we do not get the default extensions listed and also the 'Allow Nightly to make personalized extension recommendations' does not seem to affect the recommendations in any way.
In order to reproduce the issue:
Open about:config using any profile and set the 'extensions.webservice.discoverURL' pref to point to the dev environment: https://discovery.addons-dev.allizom.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%/%COMPATIBILITY_MODE%
Go to about:addons and select the Get Add-ons tab, notice the recommended extensions listed here.
Open a new Private Browsing window and go to the about:addons again
Go to options and uncheck the 'Allow Nightly to make personalized extension recommendations' checkbox located in Privacy and Security, then go back to about:addons and see the recommendations.
Step 3, 4 - The default recommended addons should be shown instead of personalized ones
We are shown the same list of recommendations regardless of the checkbox or private browsing mode. In some remote cases while in private browsing mode, I've also had recommendations change upon refreshing the page.