You can do this with karabiner-elements (https://karabiner-elements.pqrs.org/) by remapping cmd-l to cmd-d. I believe you can even add a condition to make the mapping only active in firefox, but I haven't tested that.

Here is a rule that does it (only for left command, but that's probably sufficient): https://genesy.github.io/karabiner-complex-rules-generator/#...