Staff member
If Firefox gets a Tor-protected Private Browsing mode, it will be the second major browser to do so --the first being Brave.

Ironically, even if Brave was founded by Brendan Eich, one of Mozilla's former CEOs, Mozilla can't use anything from Brave's Tor integration, as Brave runs on Chromium --the same platform on which Google Chrome has been built.
via ZDnet
1. Mozilla Research Call: Tune up Tor for Integration and Scale | Tor Blog
Integrating Tor into Firefox would bring real private browsing and a safer internet experience to an unprecedented number of people around the world. But Tor has never been deployed at this scale, and there are a lot of considerations to research before Mozilla gives this a try.
2. Mozilla Research Grants 2019H1
RQ12: Privacy & Security for Firefox
Mozilla has an interest in potentially integrating more of Tor into Firefox, for the purposes of providing a Super Private Browsing (SPB) mode for our users. Tor offers privacy and anonymity on the Web, features which are sorely needed in the modern era of mass surveillance, tracking and fingerprinting. However, enabling a large number of additional users to make use of the Tor network requires solving for inefficiencies currently present in Tor so as to make the protocol optimal to deploy at scale. Academic research is just getting started with regards to investigating alternative protocol architectures and route selection protocols, such as Tor-over-QUIC, employing DTLS, and Walking Onions