Molly was a slang term for 'pure' MDMA at least as far back as the late nineties/early aughts, from 'molecule'. The usage was to differentiate from 'pressies' which were pressed-pills supposedly composed of MDMA plus binder plus whatever the presser wanted to add, usually amphetamine though sometimes DXM or methamphetamine.

Molly, in contrast, was sold in loose powder form, at least back then.

Contrary to urban legends, pressies were never made with heroin as the amount of heroin required for any appreciable effect would be cost-ineffective.

Now, who knows what your 'molly' might contain. We just had two thirteen year-olds in the area die from 'research chemicals' ordered off the internet by a friend of theirs. People are taking all sorts of weird things that have undergone zero testing, even when compared to the embargo that MDMA has faced.