You may have noticed that Windows has had no updates for Nightly for the last week or so. We’ve had a few issues with signing the binaries as part of moving from a SHA-1 certificate to SHA-2. This needs to be done because Windows won’t accept SHA-1 signed binaries from January 1 2016 (this is tracked in bug 1079858).

Updates are now re-enabled, and the update path looks like this

older builds  ->  20151209095500  ->  latest Nightly

Some people may have been seeing UAC prompts to run the updater, and there could be one more of those when updating to the 20151209095500 build (which is also the last SHA-1 signed build). Updates from that build should not cause any UAC prompts.

https://ftangftang.wordpress.com/2015/12/11/updates-for-nightly-on-windows/