I believe Tor browser is already just a version of Firefox if I'm not mistaken. What would be the advantage of integrating with Firefox as opposed to say, a VPN integrated into the browser via a plugin. Just seems a little redundant and Tor is beginning to seem dated also with new solutions popping up and making the pitfalls of Tor more apparent.
I think an important part is to lessen the maintenance burden for the Tor project: if the code is in Firefox proper, Firefox developers will encounter it when working on other features and make sure they work together, whereas currently the Tor project needs to rebase their modifications onto "regular" Firefox.
