In Firefox 3, the answer is 6 per domain: as soon as a 7th XmlHttpRequest (on any tab) to the same domain is fired, it is queued until one of the other 6 finish.
Wha
One trick you can use to increase the number of concurrent connections is to host your images from a different sub domain. These will be treated as separate requests, each domain is what will be limited to the concurrent maximum.
IE6, IE7 - have a limit of two. IE8 is 6 if you have a broadband - 2 (if it's a dial up).