Today when I updated Nightly I noticed that there were no DevTools. In the Tools > Web Developer menu the only entry is "Page Source" Keyboard shortcuts to open the devtools don't work either, and the "Inspect" menu item in the context menu is missing as well. I'm running Windows 10 64-bit Creators Mode update. I have also heard reports from sole and dao of the same issue, on Mac and on Linux.
Something must be pretty broken with our automated testing for this to make it to Nightly.
Fixed in 56 in bug 1384967