See FireFox issue (if it's a FireFox issue) maybe it's an issue with this script
Also see recent twitter conversation
Thanks for filing this. I looked into the code and it indeed seems like a FF bug. When you refresh the page the value of performance.timing.navigationStart is not related to the actual actions on the page. It's also not equal to the performance.timing.loadEventEnd of the previous page. And it doesn't happen in Chrome.
Results in Aurora's web console https://skitch.com/kaaes/83aq6/dock
Thanks! I added your info to the bugreport. Hope someone picks this up...