Running headless Chrome or Firefox

[mgajda]

It would be nice to add --headless browser option when launching in production, available in Chrome since v59, also available in Mozilla Firefox.

[sangaline]

:+1:, we should definitely at least allow people to specify command-line arguments when launching browsers. Some other flags can also be useful in production, and it's easy to pass these through. However, I should note that Chrome unfortunately doesn't yet support web extensions when running in headless mode. Our hope is that they'll eventually add support for the parts of the API that don't depend on the UI Chrome, but the situation is uncertain. Firefox does support running in headless mode, and this is currently the default behavior.