In 2011, we introduced the concept of “preliminary review” on AMO. Developers who wanted to list add-ons that were still being tested or were experimental in nature could opt for a more lenient review with the understanding that they would have reduced visibility. However, having two review levels added unnecessary complexity for the developers submitting add-ons, and the reviewers evaluating them. As such, we have implemented a simpler approach.

Starting on August 22nd, there will be one review level for all add-ons listed on AMO. Developers who want to reduce the visibility of their add-ons will be able to set an “experimental” add-on flag in the AMO developer tools. This flag won’t have any effect on how an add-on is reviewed or updated.

All listed add-on submissions will either get approved or rejected based on the updated review policy. For unlisted add-ons, we’re also unifying the policies into a single set of criteria. They will still be automatically signed and post-reviewed at our discretion.

We believe this will make it easier to submit, manage, and review add-ons on AMO. Review waiting times have been consistently good this year, and we don’t expect this change to have a significant impact on this. It should also make it easier to work on AMO code, setting up a simpler codebase for future improvements.  We hope this makes the lives of our developers and reviewers easier, and we thank you for your continued support.

https://blog.mozilla.org/addons/2016/08/19/a-simpler-add-on-review-process/