It would be a fantastic edition to "private" mode. Also, by being baked into the browser, it would give Mozilla a degree of control over the protocols passed. A user opting for the browser plugin instead of the standalone client would not be passing Bittorrent traffic over the network.
More relays and endpoints would be needed, but I wonder how kind of support could be gathered if Tor was publicized in such a way.
Having it built in, and defaulting to On when you open a private browsing window would be nice though.