keepee I can reproduce it every time I start the demo. It isn't even needed to do anything. Just running the demo. Within less than 20 seconds I can see FF's RAM load raising from 300 MB to over 1 GB (at that point I stopped the tests), and most of the acquired RAM isn't released after closing the page. A FF restart is needed. Now that you described, how you did it, I wonder if it is the canvas plugin having issues? Maybe I should alert rojohound just to make sure.
EDIT: I just tested it with Chrome 29 with the same issue. So, if I'm not the only one working with WinXP 32bit and one of these two browsers, there has to be others experiencing the same. It's just that you are not aware of it when running the demo for just a minute or so (assumed, most people will have more than 2 GB RAM)