Support --pause-on-error in Firefox

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](https://wiki.mozilla.org/Remote_Debugging_Protocol). 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.

Start by reading the Firefox Remote Debugging Protocol documentation and reviewing issue #294 and the Observatory workflow. Determine whether the protocol can support the requested pause-on-error behavior; done means the command-line flag works reliably when running tests in Firefox.

Written by the indexing model from the issue text.