An Italian Windows blog has found the first known extension for Edge, the default browser on Windows 10. The description of the extension indicates also Edge for Windows 10 Mobile will support extensions.
The extension is called Page Analyzer and can’t be found on the regular Windows Store. It was already known that Edge would support extensions, likely compatible with those of Google’s Chrome browser.
The upcoming Redstone update for Windows 10 should bring extensions to Edge and it appears they can be downloaded from the Windows Store as apps. According to Windows Italia extensions are installed just like apps and are also added to the start screen. When the extension is started the Edge browser is started.
The Italian site also notes that the extension has support for ARM support which indicates that Edge on Windows 10 Mobile might also support extensions.