I tried it on Chome, Firefox and IE with similar results. (and Firefox behaves even weirder with the player sprite moving very strangely)
Also, I saw this tread: http://www.scirra.com/forum/jump-thru-platform-positive-x-movement_topic53067.html
...and used the updated platform runtime.js with similar results.
Here are my specs:
Windows Vista Home SP2
GEForce GTX 560