Hey, I wanted to thank you again. I tested on Firefox, Chrome and Safari (my default). Both Chrome and Safari peg at 60 fps, as they should, but I did notice some lag (around 55 fps on my very fast machine) in Firefox. I also noticed that switching off the music and sound fx (M and S keys, or in the options) helped. I also noticed that the longer it ran in Firefox, the slower it seemed to run (it dropped down to 52 fps on my machine after quite some time).
If you go into the options and press D, there are some debug numbers shown. The top right number is the fps. I wonder what you get in Firefox.