The term MMR:DE refers to a specific Lode Runner project, currently in active development. It's an unofficial built-from-scratch Mad Monks' Revenge (or Lode Runner On-Line) project by QuarkRobot, found here: https://mmr.quarkrobot.com/. The link is also in the Resources tab.

MMR:DE is probably the easiest way to play the game without downloading emulators or abandonware. MMR itself contains the single player levels of TLR, plus additional levels exclusive to MMR, but does not contain the TLR exclusive Co-op levels.

MMR:DE is only allowed for Single Player runs of Lode Runner: The Legend Returns for the above reason. Be sure to mark "DE" for the "MMR:DE?" option when submitting a run.

This thread will explain the glitches discovered in TLR. http://entropymine.com/jason/lr/

Floating Item Usage: When exiting a ladder to the side, a player can use the gas gun (other items?) on the frame the player should fall. For as long as the player continues using the gas gun, the player will float indefinitely in the air.

Instant Block: When falling rocks from the pickaxe collide with a falling monk or falling player, the player or monk become suspended in the air forever and become obstructing blocks. Players and monks can stand on and cannot move through a frozen monk or player.

Co-op Infinite Lives: Restarting the level decreases the life count of both players. If one player has zero lives and the level is restarted, the player revives at -1 lives. So long as the player does not complete a level and gain lives back to 0, the player will continue to spawn into a level and play normally.