Having a wide variety of content on addons.mozilla.org has always been a top priority for us, which is why some years ago we decided that we needed two different levels of acceptance for the add-ons we host and distribute. Fully reviewed add-ons meet all of our quality standards, while preliminarily reviewed add-ons only have to meet a lower standard, focused mostly on security and privacy. This gives developers the liberty to distribute experimental and not-quite-ready add-ons, and gather user feedback as they work on them.
Naturally, we make both review levels easily distinguishable on AMO, giving fully reviewed add-ons the usual green install button, while preliminarily reviewed add-ons have a stripped yellow button. We also give fully reviewed add-ons higher precedence on search results and most other places.
Unfortunately, we never implemented this distinction in the Firefox Add-ons Manager. This meant that users were able to find and install preliminarily reviewed add-ons without any indication of their review level. Since we believe this can be detrimental for the majority of our users, we decided to limit Add-ons Manager search results to fully reviewed add-ons. This is a server side change, so it applies to all versions of Firefox.
If you’re interested in trying more experimental add-ons, you can always find them on AMO.