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+======================
+Firefox Home (New Tab)
+======================
+
+All files related to Firefox Home, which includes content that appears on ``about:home``,
+``about:newtab``, and ``about:welcome``, can be found in the ``browser/components/newtab`` directory.
+Some of these source files (such as ``.js``, ``.jsx``, and ``.scss``) require an additional build step.
+We are working on migrating this to work with ``mach``, but in the meantime, please
+follow the following steps if you need to make changes in this directory:
+
+For ``.jsm`` or ``.sys.mjs`` files (system modules)
+---------------------------------------------------
+
+No build step is necessary. Use ``mach`` and run mochitests according to your regular Firefox workflow.
+
+For ``.js``, ``.jsx``, ``.scss``, or ``.css`` files
+---------------------------------------------------
+
+Prerequisites
+`````````````
+
+You will need the following:
+
+- Node.js 10+ (On Mac, the best way to install Node.js is to use the install link on the `Node.js homepage`_)
+- npm (packaged with Node.js)
+
+To install dependencies, run the following from the root of the mozilla-central repository.
+(Using ``mach`` to call ``npm`` and ``node`` commands will ensure you're using the correct versions of Node and npm.)
+
+.. code-block:: shell
+
+  (cd browser/components/newtab && ../../../mach npm install)
+
+
+Which files should you edit?
+````````````````````````````
+
+You should not make changes to ``.js`` or ``.css`` files in ``browser/components/newtab/css`` or
+``browser/components/newtab/data`` directory. Instead, you should edit the ``.jsx``, ``.js``, and ``.scss`` source files
+in ``browser/components/newtab/content-src`` directory. These files will be compiled into the ``.js`` and ``.css`` files.
+
+
+Building assets and running Firefox
+-----------------------------------
+
+To build assets and run Firefox, run the following from the root of the mozilla-central repository:
+
+.. code-block:: shell
+
+  ./mach npm run bundle --prefix=browser/components/newtab && ./mach build && ./mach run
+
+Continuous development / debugging
+----------------------------------
+Running ``./mach npm run watchmc --prefix=browser/components/newtab`` will start a process that watches files in
+``activity-stream`` and rebuilds the bundled files when JS or CSS files change.
+
+**IMPORTANT NOTE**: This task will add inline source maps to help with debugging, which changes the memory footprint.
+Do not use the ``watchmc`` task for profiling or performance testing!
+
+Running tests
+-------------
+The majority of New Tab / Messaging unit tests are written using
+`mocha <https://mochajs.org>`_, and other errors that may show up there are
+`SCSS <https://sass-lang.com/documentation/syntax>`_ issues flagged by
+`stylelint <https://stylelint.io>`_.  These things are all run using
+``npm test`` under the ``newtab`` slug in Treeherder/Try, so if that slug turns
+red, these tests are what is failing.  To execute them, do this:
+
+.. code-block:: shell
+
+  ./mach npm test --prefix=browser/components/newtab
+
+These tests are not currently run by ``mach test``, but there's a
+`task filed to fix that <https://bugzilla.mozilla.org/show_bug.cgi?id=1581165>`_.
+
+Windows isn't currently supported by ``npm test``
+(`path/invocation difference <https://bugzilla.mozilla.org/show_bug.cgi?id=1737419>`_).
+To run newtab specific tests that aren't covered by ``mach lint`` and
+``mach test``:
+
+.. code-block:: shell
+
+  ./mach npm run lint:stylelint --prefix=browser/components/newtab
+  ./mach npm run testmc:build --prefix=browser/components/newtab
+  ./mach npm run testmc:unit --prefix=browser/components/newtab
+
+Mochitests and xpcshell tests run normally, using ``mach test``.
+
+Code Coverage
+-------------
+Our testing setup will run code coverage tools in addition to just the unit
+tests. It will error out if the code coverage metrics don't meet certain thresholds.
+
+If you see any missing test coverage, you can inspect the coverage report by
+running
+
+.. code-block:: shell
+
+  ./mach npm test --prefix=browser/components/newtab &&
+  ./mach npm run debugcoverage --prefix=browser/components/newtab
+
+Detailed Docs
+-------------
+.. toctree::
+  :titlesonly:
+  :glob:
+
+  v2-system-addon/*
+
+..  _Node.js homepage: https://nodejs.org/