User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20160210153822 Steps to reproduce: 1) Log in to Windows 10 using a Standard user account. Mine was a Local account, not a Microsoft account. 2) Run the Firefox installer. 3) A UAC prompt is shown. Click No. Firefox is installed to C:\Users\...\AppData\Local\Mozilla Firefox. 4) Close Firefox. 5) Use Windows' Start menu -> Settings -> System -> Apps & features to start the uninstaller. 6) A UAC prompt is shown: 6a) If I click No, the uninstaller is not run. 6b) If I type in an administrator user's password (also a Local account, not a Microsoft account) and click Yes, the uninstaller runs to completion but does not uninstall Firefox completely. The desktop short-cut is still present, Firefox still shows in Apps & features, and the registry keys under HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Mozilla Firefox 44.0.2 (x64 en-GB) are still present. At step #5, if I use Control Panel -> Programs -> Programs and Features instead, a UAC prompt is still shown, but clicking No starts the uninstaller and the program can be completely uninstalled. Windows 10 v1511 Build 10586.0 64-bit. Firefox 44.0.2 64-bit.

This is still reproducible, and it's been made worse in Windows 10 version 1703, because Apps & Features has replaced the Programs & Features control panel on the Win+X menu, making it the easier UI to access.

I think this is a Windows bug, and there doesn't seem to be any workaround. The Settings app appears to be unconditionally trying to elevate before starting the uninstaller process, and it simply refuses to start the process if the elevation is denied/unsuccessful. This is always its behavior, for every app, not just for us; other applications installed without elevation get the same behavior and end up presenting the same bug. The old Programs & Features Control Panel simply starts the process using Explorer's token, and lets the uninstaller elevate itself if it wants, which is why it handles this situation correctly. I can't imagine why the Win10 Settings app seems to assume that every uninstaller requires elevation; it seems like an obvious bug. And if there's any way of overriding that behavior, I can't find it and it isn't documented. So I can't do anything about this unless Microsoft fixes something.