[tedks]

That is precisely the topic of the article. For a more in-depth view, you can read this tor-talk mailing list thread: http://archives.seul.org/or/dev/Sep-2014/msg00091.html

(This is the original source for the Dot article.)

A summary: Raw bandwidth isn't the limiting factor in scaling Tor; before you just plug in traffic to the network, you'd need to optimize Tor's internal protocols to make expansion of the network even possible. However, those hurdles are fairly small, and once they're handled, it's just a matter of funding to pump up more nodes, which is much more simple.

Further, not all Firefox users (if the article's speculation is correct) will be using private browsing at once. So the actual increase is probably 5-10% (at most? anyone know actual statistics about private browsing?) of that number.