Needing to retype every single long command one has already entered once inside xpcshell is just plain annoying.
xpcshell has the ability to use GNU readline (search for JS_READLINE in js/src/xpconnect/shell/Makefile.in). I don't think we are allowed to automatically define the variable because of licensing issues. We could pull js/src/editline into the regular build and use that by default, though.
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