Actually, we might be able to verify that this is the culprit. Gian-Carlo, does the build start if you manually replace /Applications/FirefoxNightly.app/Contents/Resources/defaults/pref/channel-prefs.js with the channel-prefs.js file from a fresh download of Nightly?
Bug 1799332 Comment 12 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Actually, we might be able to verify that this is the culprit. Gian-Carlo, does the build start if you manually replace /Applications/FirefoxNightly.app/Contents/Resources/defaults/pref/channel-prefs.js with the channel-prefs.js file from a fresh download of Nightly? And does `codesign -vvvv` now show that the app bundle is properly signed?