Support --pause-on-error in Firefox

[nex3]

This is a very useful flag for debugging. With #294 it isn't strictly necessary, since the user can just set it in Observatory, but it's much easier to set it via the command-line.

This will require connecting to the Firefox dev tools. It seems to have a much more complex protocol than the Chrome dev tools, so it's likely that this will be more effort than it's worth for the near future.