Official coffee repl toggles multi-line input with CTRL-V. That is first CTRL-V switches on multi-line input. Second CTRL-V gets out of multi-line input.
I think mozrepl should follow the same philosophy to be compatible with the standard coffee repl implementation. This becomes especially important when using mozrepl with external programs that rely and are designed to work with the coffee repl. One such example is Vim and Slimux.