In Tor Browser 4.5a5, we decided to increase MaxCircuitDirtiness to 2
hours (https://trac.torproject.org/projects/tor/ticket/13766).

Because we also use Tor's SOCKS username isolation using the URL bar
domain as the SOCKS username in Tor Browser 4.5 now, this has the effect
that websites you visit will continue to use the same circuit (and thus
the same exit IP) for all of their content elements for 2 hours, or
until you click "New Identity" or "New Tor Circuit for this Site" (which
appeared in Tor Browser 4.5a4).

The reasons for this change are detailed in that ticket description, but
in summary I think it is a really, really bad user experience when a
website switches languages, bans you, or logs you out every 10 minutes.
My own workflow in Tor Browser has been frequently interrupted by this
in ways that have caused lost work and/or lost access due to this
problem.

With this change in combination with the "New Tor Circuit for this Site"
Torbutton menu option, you now have the ability to get a good circuit
for a site that you can actually use long enough to get something done.

However, there are some downsides to this change:

1. Longer circuit lifetimes may mean more memory consumption at relays.
2. Longer circuit lifetimes may make correlation easier for adversaries
   that run Tor nodes or can see inside TLS (by stealing node keys).
3. Longer circuit lifetimes may distinguish Tor Browser users at the
   Guard node.
4. Longer circuit lifetimes may mean that the Tor client is less able
   to adapt to transient changes in Tor relay overload - the load
   conditions that caused the Circuit Build Timeout code to pick
   your current path may have long since changed over the span of 2
   hours.
5. We actually had to hack update, OCSP, and favicon requests to
   continue to use 10 minute circuits, because Firefox does not make it
   easy to determine the URL bar associated with them. (We opted to keep
   the circuits for these requests at 10 minutes to avoid excessive
   linkability at the exit.)

Did I miss any?

Long term, I think what I'd rather do to achieve this functionality is
to create a "TrackIsolationExits" Tor feature that ensures that Tor
Browser keeps the same exit IP for a given URL bar domain independent of
the underlying circuit lifespan, as I mentioned in
https://trac.torproject.org/projects/tor/ticket/15458#comment:2.

So: How do we make the decision as to if it is more important to improve
usability in the short term while we design and implement
"TrackIsolationExits", or if the above concerns (and others) trump poor
usability?

Right now, I am inclined to make the choice that leads to more people
being able to effectively use Tor Browser in the short term, and then
try to provide a better solution that gives similar user-facing
behaviors with better network usage properties in the long term.

To complicate matters, as ticket #15458 indicates, there are several
other security concerns related to circuit use by Tor Browser that also
need to be ironed out. In particular, it is actually somewhat dangerous
to allow websites to use a new circuit every 10 minutes for things like
Javascript/AJAX requests, because this enables Guard discovery. SOCKS
isolation and a long circuit lifespan may actually make such Guard
discovery harder, but unfortunately, there may still be other ways to do
this in Tor today (See
https://trac.torproject.org/projects/tor/ticket/13669 and
https://trac.torproject.org/projects/tor/ticket/7870).

Thoughts?