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:

% ./mach marionette-test --gecko-log -

It is common to use this in conjunction with an option to increase the Marionette log level:

% ./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.

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:

% ./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:

    GeckoDriver.prototype.getPageSource = async function() {

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:

% ./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.

  • → 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.