Here's what they listed for 70.0a1 back in July, but the release notes link for today's update is a dead end.
https://www.mozilla.org/en-US/firefox/70.0a1/releasenotes/
One of the main changes for me as a Mac user is this: https://bugzilla.mozilla.org/show_bug.cgi?id=1429522#c32
Now I'd be losing the tabs/bookmarks integration with iOS's unchangeable default browser, but that's not as big a deal to me.
Here's what they listed for 70.0a1 back in July, but the release notes link for today's update is a dead end.
https://www.mozilla.org/en-US/firefox/70.0a1/releasenotes/