I create a method to post json data to web service :
function WishList() { }
WishList.prototype.addToWishList = function(redirectURL, postURL, userObj) {
Not sure what you are using as the service on the other end but this may be due to cross domain posting. I hate to post a link and run but this may be of some use to you.
http://praneeth4victory.wordpress.com/2011/09/29/405-method-not-allowed/
Looks like they could get it working in IE but had some issues as you with the other browsers. Perhaps these couple changes will help access the service better.
This post was good at explaining the error and parts to it as well so if the above link is not helpful this one may help you diagnose the issue further.
http://www.checkupdown.com/status/E405.html
ok ok last edit, just wanted to make sure you have enough info to hopefully resolve your issue, here is a good article on the underlying problem I believe you are having..
http://www.d-mueller.de/blog/cross-domain-ajax-guide/