This is your 1st platformer ? The jump look great but there is some performance issue on Firefox( I think it comes from monster, when they are all dead this is fine). Some feedback: Your art is good but would benefit to use the same pixel size (background, monster, blocks and user have different pixel size). Your main character is too bright in comparison of all the rest. The arms is sometime not at the right place. The cool down of the crossbow is perhaps too long.
Continue, It's a good start !