Molten Redstone is a liquid added by the Thermal Expansion mod. It is used for making Redstone energy cell and Redstone Energy Conduits.
You make molten redstone by melting redstone dust in a Magma Crucible. The exact recipe has changed a few times. Before TE v2.0.7 (FTB BETA A pack uses 2.0.5) the Redstone recipe was 1000 MJ for 50mB; with that version is was changed to 1500 MJ per 50 mB; in v2.1.5 the recipe changed to 1200 MJ for 25mB per piece of redstone dust.