AJAX跨源

Question

I want to make AJAX call to cross domain, and the cross domain being REST web service hosted on a remote host. Due to the restriction in the work environment and my personal interest, I am writing this in pure Javascript (not jQuery) with XHR. It works fine on IE11 but not Firefox/Chrome. The error on Firefix/Chrome is due to the same origin policy. Interestingly, if I open the REST based URL on those two browsers, the correct respond comes back without any block from the same orgin policy. What is the difference between making AJAX call and opening it in browser when it comes to the restriction of the same origin policy? I came across to CORS, but withCredentials won’t help. I cannot modify browser setting due to the restriction of the work environment. My purpose is to make cross browser compatibility if possible. Thank you!