The preference is no longer needed. Necessary breakpoint could be set in the Marionette code before starting Firefox by adding debugger statements. Right now it's only annoying to have to click away those modal dialogs.

Actually there is also marionette.contentListener, which is marked as deprecated:

https://searchfox.org/mozilla-central/rev/44695ef057e422a8d6c6056972bdf32766c36187/modules/libpref/init/all.js#4556-4558

Both got added via bug 1456050 and don't seem to be needed anymore.

Actually it would be good to keep the marionette.debugging.clicktostart preference but don't set it implicitly to true when --jsdebugger is passed. That would still allow users of Selenium / geckodriver to debug a Marionette session without having to download the code first.

Always getting a user prompt shown when debugging code in Marionette
is disturbing. Given that we usually use the "debugger;" statement
anyway, this prompt will not be useful by default.

But we should keep it to actually allow users of Marionette and
geckodriver to run a debug session without having to modify the
code first.

Depends on D107428

https://hg.mozilla.org/mozilla-central/rev/8e9fa8d73a47
https://hg.mozilla.org/mozilla-central/rev/fdae009d0b38