I wrote a post about this at http://www.shanison.com/2010/05/10/stop-the-browser-%E2%80%9Cthrobber-of-doom%E2%80%9D-while-loading-comet-forever-iframe/
Below are summary solutions for different browsers:
1. Firefox
In firefox, before you trigger the call back method to update the content, you have to create a temp iframe and attach to the document body, then removes it. By keep doing this before you trigger the event, the browser throbber goes away.
2. Chrome/Safari
For chrome and safari if you just do like above, the forever iframe won’t work at all! There won’t be any data returned from server side. So how do you make comet iframe work in chrome and safari.The hacks lies on server side. There are two conditions that you must fullfill in order to make forever iframe happens in this webkit browsers.
Firstly, the server must push some junk data (around 2k) to the browser before you push the real data. So just write out some javascript comments to the browsers first.
3. IE
In IE, if you use above iframe method, it works. I have successfully done that. However in order to stop the browser throbber of doom, you have to make use of the special htmlfile object.