Firefox 70 release notes: https://www.mozilla.org/en-US/firefox/70.0/releasenotes/

Source: https://ftp.mozilla.org/pub/firefox/releases/70.0/

CVEs Fixed (omitting moderate and low):

CVE-2018-6156: Heap buffer overflow in FEC processing in WebRTC
CVE-2019-15903: Heap overflow in expat library in XML_GetCurrentLineNumber
CVE-2019-11757: Use-after-free when creating index updates in IndexedDB
CVE-2019-11764: Memory safety bugs fixed in Firefox 70 and Firefox ESR 68.2

New Features:
Social Tracking Protection included in the Enhanced Tracking-Protection
Privacy Protections Report (Overview of Enhanced Tracking Protection)
Faster Javacscript Baseline Interpreter