The currently builds are version 67 I believe so the installer needs to be updated to grab the new builds. Currently it just fails to download.
You are here
Firefox Nightly installer needs to be updated
February 10, 2019 - 11:54pm
#1
Firefox Nightly installer needs to be updated
February 11, 2019 - 9:14am
#2
Outdated Apps Thread
Please report outdated apps in the Outdated Apps Thread: https://portableapps.com/development/outdated
You can find the link from the Support and Development page as well as a large header announcement when creating a new forum thread.