While performance in IE, Firefox on Windows, and Safari was acceptable (the performance of Safari has been running circles around the other browsers), performance on Firefox on Mac was incredibly poor. Amazingly so.
After the initial panic, I set to tracking the precise cause of the performance issues. By commenting out large sections of the code, I was able to determine that we were calling offsetHeight on some DIVs repeatedly each time an event fired (and it fires a lot).
While I can understand why offsetHeight is slow, I don't understand why the performance of it in Firefox on Mac (Macbook Pro) was so much worse than any other browser.