Ideally, you should be checking to see if the image you're switching in has completed loading before trying to execute any further logic on the page, but I've put together a working test case based on what you mention in your ticket here http://jsfiddle.net/addyosmani/HSg7N/. Checking with Firefox 3.6 (and also 3.5 just for posterity) this is working absolutely fine for me. The issue you experienced may have either been down to 1) The fact you're missing '});' at the end of your code or 2) A slow server serving up your images. If 2) is the case, please refer to my initial advice regarding adding a load handler to see if the image has in fact finishing loading up.