I updated to the latest nightly after not doing so for at least two months. After starting Fennec, I saw a black screen with the spinner at the top and the text "Fennec". After a few seconds this disappeared and I was back to the desktop. I tried to open Fennec a couple more times, with the same result, so I opened the terminal to see if there was any output. While the terminal was opening, suddenly the Fennec screen appeared.