Bug 1052211 added an --auto-close option, but bug 1155338 seems to have removed all mention of the option. The option no longer works for me. Probably this was a mistake?

It was actually on purpose, but there's an argument for adding it back in. I tried to make --auto-close the default behaviour *all* the time, and then --keep-open could be used if needed. But there's an exception to the rule when running a single mochitest-plain test where the browser doesn't get automatically closed. I think this is more or less a dupe of bug 1185761, see that bug for a more thorough explanation of the situation.