Searches in the Firefox in browser Add-ons Manager (Tools > Add-ons) will no longer return results for add-ons which are experimental or not “Fully Reviewed”.
Fully reviewed add-ons meet all of our quality standards, while preliminarily [sic] reviewed add-ons only have to meet a lower standard, focused mostly on security and privacy.
While Mozilla did make the reviews levels clear on the AMO site, the in browser add-ons manager did not show a distinction between Fully Reviewed and Experimental add-ons. As a result, users could unknowingly install an experimental add-ons when they believed they were installing a Fully Reviewed add-on. Users can still search for and install experimental add-ons via the AMO (addons.mozilla.org) site.