To ensure the highest quality experience for customers, and as we have done with Windows 10 since the beginning, we will introduce extensions first to our Windows Insiders and then roll them out more broadly to all Windows 10 users. Initially our Insiders will download and then sideload extensions for testing purposes, but ultimately we will make extensions available to all of our customers via the Windows Store to ensure the quality of the end-to-end experience. Microsoft Edge to Get Browser Extensions With Insider Preview [blogs.windows.com]