Microsoft released an "accidental" preview site for the upcoming Edge browser extensions.
The release was a preview for a developer site which announced Edge browser extensions support.
Indications from the preview site are that developers will have the chance to write extensions ahead of the general release, and Windows 10 testers would have the ability to try the extenstions out ahead of time as well.
Microsoft had originally planned to release extensions for the Edge browser in a patch release of the Windows 10 operating system, but delays caused the extensions release to be moved to early 2016.
It is likely that the extensions would leverage the Windows Store, which is integrated with Windows 10.
The Edge browser was released with Windows 10 initially, and is not available for previous versions of Windows.
From the screenshot of the extensions site, it appears that extensions for Reddit and Pinterest are available for download.
Details of the extensions preview site from Microsoft can be found on Twitter here
More information about Edge browsers is available on our Edge browser user agent strings page.