The `NIGHTLY_BUILD` flag is used in mozilla-central to enable some useful developer and debugging features. However, it is not set when building tor-browser because it is based on "a1" being part of `version.txt`. Rather than switching on the `NIGHTLY_BUILD` flag ourselves and enabling everything, we could add our own flag `TOR_BROWSER_NIGHTLY_BUILD` and use it to selectively opt in to some of the features. We can base this on the update channel, but unlike `MOZ_UPDATE_CHANNEL`, we this will just be a binary flag so can be used in the C++ preprocessors.