Motivation
----------

Preparing the switch to a new Firefox ESR begins usually as soon as it lands on
Mozilla's beta or release channel. This means for us we have 12 to 18 weeks to
prepare a Tor Browser release based on it. This includes adapting our Gitian
setup, rebasing and testing our old patches, auditing the network code,
reviewing new features and modifying them to fit our needs, testing them in at
least one alpha release etc.

While there are things that are tricky to do without additional overhead before
a new ESR got out, reviewing new features, prioritizing them and filing tickets
is not necessarily one of those.

Feature review process
----------------------

To illustrate the workflow of the feature review process let's assume we just
got Tor Browser based on Firefox ESR 38 out of the door. Mozilla released
Firefox 40 meanwhile. One of the Tor Browser team members is catching up on the
Firefox for developers documentation for version 39 and 40 and is filing bugs
with the "ff45-esr" tag. Reviewing the developer documentation and filing
respective bugs is happening successively after Firefox 41-44 get out as well.
This procedure is covering the most important and most difficult to patch things
and should therefore be enough for the first pass. As a bonus point we might
even get hints about possible problems with code already in the ESR 38 series
this way which would allow us acting quicker in fixing those issues in our
stable series.

When we finally start preparations for the switch to ESR 45 we basically have
all the ff45-esr tickets filed, and ideally prioritized, which are resulting
out of Firefox 39-44 reviews. The only remaining tasks for the feature review
are: taking care of ESR 45 and double-checking the undocumented bugs and the
ones with milestone Firefox39-45. Additionally, it is worth comparing the
preference changes by diffing `all.js` and `firefox.js` to make sure all
features we want to be disabled are still disabled and things we want to have
enabled are so in fact. Checking the diff between those preference files might
as well help finding issues we overlooked while doing the feature review
outlined above.
