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+# Debugging
+
+## Redirecting the Gecko output
+
+The most common way to debug Marionette, as well as chrome code in
+general, is to use `dump()` to print a string to stdout.  In Firefox,
+this log output normally ends up in the gecko.log file in your current
+working directory.  With Fennec it can be inspected using `adb logcat`.
+
+`mach marionette-test` takes a `--gecko-log` option which lets
+you redirect this output stream.  This is convenient if you want to
+“merge” the test harness output with the stdout from the browser.
+Per Unix conventions you can use `-` (dash) to have Firefox write
+its log to stdout instead of file:
+
+```shell
+% ./mach marionette-test --gecko-log -
+```
+
+It is common to use this in conjunction with an option to increase
+the Marionette log level:
+
+```shell
+% ./mach test --gecko-log - -vv TEST
+```
+
+A single `-v` enables debug logging, and a double `-vv` enables
+trace logging.
+
+This debugging technique can be particularly effective when combined
+with using [pdb] in the Python client or the JS remote debugger
+that is described below.
+
+[pdb]: https://docs.python.org/3/library/pdb.html
+
+## JavaScript debugger
+
+You can attach the [Browser Toolbox] JavaScript debugger to the
+Marionette server using the `--jsdebugger` flag.  This enables you
+to introspect and set breakpoints in Gecko chrome code, which is a
+more powerful debugging technique than using `dump()` or `console.log()`.
+
+To automatically open the JS debugger for `Mn` tests:
+
+```shell
+% ./mach marionette-test --jsdebugger
+```
+
+It will prompt you when to start to allow you time to set your
+breakpoints.  It will also prompt you between each test.
+
+You can also use the `debugger;` statement anywhere in chrome code
+to add a breakpoint.  In this example, a breakpoint will be added
+whenever the `WebDriver:GetPageSource` command is called:
+
+```javascript
+    GeckoDriver.prototype.getPageSource = async function() {
+      debugger;
+      …
+    }
+```
+
+To be prompted at the start of the test run or between tests,
+you can set the `marionette.debugging.clicktostart` preference to
+`true` this way:
+
+```shell
+% ./mach marionette-test --setpref='marionette.debugging.clicktostart=true' --jsdebugger
+```
+
+For reference, below is the list of preferences that enables the
+chrome debugger for Marionette.  These are all set implicitly when
+`--jsdebugger` is passed to mach.  In non-official builds, which
+are the default when built using `./mach build`, you will find that
+the chrome debugger won’t prompt for connection and will allow
+remote connections.
+
+* `devtools.browsertoolbox.panel` -> `jsdebugger`
+
+  Selects the Debugger panel by default.
+
+* `devtools.chrome.enabled` → true
+
+  Enables debugging of chrome code.
+
+* `devtools.debugger.prompt-connection` → false
+
+  Controls the remote connection prompt.  Note that this will
+  automatically expose your Firefox instance to localhost.
+
+* `devtools.debugger.remote-enabled` → true
+
+  Allows a remote debugger to connect, which is necessary for
+  debugging chrome code.
+
+[Browser Toolbox]: /devtools-user/browser_toolbox/index.rst