They're not talking about giving Firefox a "tor mode". The Tor browser uses Firefox under the hood with a few add-on and flipped default settings. Currently, it uses a fork of Firefox with slight modifications (like the ability to disable mathml). They intend to make it so that this fork is unnecessary. The Tor browser will probably continue to be something you need to install to use.
It's almost clickbait.
Consider this. Firefox peaked a while ago; since then, Chrome ate every other browser's lunch, and the only browser that had user share declining faster than Firefox was IE (but even so, IE still has more users). Google has the brand recognition on their side, and on top of that, they have actually made a browser that "just works" for 99% of people out there, the way Firefox never quite did.
Since contesting Chrome's market share is rather pointless, Firefox needs a niche of its own now. Integrating Tor in a seamless way would be a great opportunity to create one.
multiprocessing to stop freezing issues and increase response speed
a permissioned API for extensions, easier and saner full themes system (faster/easier development time for updates, and a wider range of extensions/themes as a result)
and switching critical components to a safer language)
which imo are more important for users than doing something an existing firefox fork does (like, if there was demand for firefox with seamless tor, then they would get the firefox with seamless tor today, because it already exists)