After being shut out of one of the most popular sites on the Internet last week, many non-Microsoft browsers on Monday were finally able to access the software giant's MSN.com page. Some Mozilla and Opera users were enraged Thursday when they could not reach the upgraded MSN site. Instead, they were given the option of downloading a version of Microsoft's Internet Explorer. That same day, Microsoft buckled to protests and said it would open its site to other browsers. But the uproar spilled into Friday as many Mozilla and Opera users found themselves still locked out. The episode also raised the ire of Washington-based trade group ProComp, which asked state and federal trustbusters to get involved.