It has a "What's new" link next to the version number, but it links to a 404: https://www.mozilla.org/en-US/firefox/70.0beta/releasenotes
EDIT - I put in a bug report on the release notes link, response says that release notes will come out next week
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.
It has a "What's new" link next to the version number, but it links to a 404: https://www.mozilla.org/en-US/firefox/70.0beta/releasenotes
EDIT - I put in a bug report on the release notes link, response says that release notes will come out next week