Using the new Firefox profiler [0], which isn't quite ready for public consumption yet but already very helpful:
1. Download Firefox Nightly [1]
2. Install the profiler addon [2]
3. Play the game
4. Press Cmd+Shift+O to open a profile of the most recent few seconds
5. Wade through the tree until you find nsIDOMHTMLImageElement_SetSrc, which is called by <Anonymous>() @ CompletedEnemies.js:2460. This function contains only one line which assigns to the src property of an image, and that's the line I pasted.