The more unstable Nightly development channel is meant more for testers and developers in mind as it can get checkins with updates twice a day. This can lead to issues until fixed, finished or reverted. https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/ One place to look if you want to be a Nightly tester is the Build threads in https://forums.mozillazine.org/viewforum.php?f=23 though the poster does not keep them updated as frequently now.