Industry groups say the trend could undermine business models that currently support many popular online services.
Some experts said they feared the conflict would lead to Internet companies creating even more sophisticated tools for tracking people as they moved around the Web.
"We're at the risk of an arms race here," Peter Swire, a Clinton administration privacy expert who is now an Ohio State University law professor, told the Post. "This could break the Internet. It interferes with existing browsing modules, and it puts bigger pressure on users to take escalating steps to protect their privacy."
Mozilla's new cookie restrictions are included in a version of Firefox released to about 10,000 users for testing, Harvey Anderson, company vice president and general counsel, said.
Anderson cited research suggesting 68 percent of people using the Internet in 11 countries say they would want to limit tracking of their Web activities if the capability were available.
"This kind of feature creates a Web that's more in line with a user's expectations," Anderson said.