I would imagine most users of Tor are using Tor Browser. I am reading there was a responsible disclosure to Mozilla but is it me or did that section leave out when the Tor Project planned to respond or release a fixed Tor Browser? Do they like keep very close or is there a large lag?
Not sure about "most". I use tor without a tor browser, because I don't care about being identified. I only used it to go around geoblocking and visit onion sites.
Tor Browser is always quick to rebase on the latest Firefox ESR. They released an update the next day:
https://blog.torproject.org/new-release-tor-browser-15010/