Following up on the general release of Windows 10 Mobile to older Windows Phones earlier today, Microsoft has pushed out a fresh build to PC and Mobile Insiders on the Fast ring. Coming in at build number 14291, the latest Insider build finally brings the addition of extensions in Microsoft Edge and much more. From Insider chief Gabriel Aul:

Today we are releasing Windows 10 Insider Preview Build 14291 for both PC and Mobile to Windows Insiders in the Fast ring. There are some very cool new things in this build, including one of the top requested features by Windows Insiders – extensions for Microsoft Edge!

Earlier today, we began the roll-out of Windows 10 Mobile to select Windows 8.1 phones. As we stated previously, devices which are eligible to receive Windows 10 will also be able to get preview builds from the Development Branch. We've already delivered a few builds to the devices which shipped with Windows 10, including the Lumia 550, 650, 950, 950 XL, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. We have updated the list of devices that will be capable of receiving updates as part of the Windows Insider Program going forward, and in the coming weeks we will deliver Windows 10 Mobile Insider Preview builds from our Development Branch to these devices in the Fast ring.