The idea is that the Tor Browser would be standard Firefox with certain strong privacy settings enabled. i.e., the Tor project wouldn't have to patch Firefox source and recompile because their patches would already be in Firefox, just run-time disabled. They wouldn't have to maintain their own set of patches that could be broken by Firefox development. For Tor, stock Firefox would be launched with those (strong privacy) settings enabled.
Tor might have to rework their patches to live in Firefox in this way, but it would simplify their work because Firefox development wouldn't cause upstream breakage of the Tor browser.