At issue is the , designed to reduce the effectiveness of an antipiracy campaign by DHS's Immigration and Customs Enforcement (ICE) division. When users try to visit a website whose Internet domain has been seized by ICE, Mafiaa Fire redirects them to a working site set up to replace the seized domain.

That's a problem for the DHS, which is trying to knock these sites offline permanently. "The ICE Homeland Security Investigations unit alleged that the add-on circumvented a seizure order DHS had already obtained against a number of domains," wrote Harvey Anderson, vice president and general counsel for Mozilla,

In recent months, ICE has shut down a large number of websites suspected of selling illegally copied music, movies or counterfeit products. Some free-speech experts have said the seizures may violate freedom-of-speech protections in the U.S. Constitution.

The DHS did not come to Mozilla with a court order, and the group , or at least a legal reason justifying the removal of the add-on.

"To date we've received no response from Homeland Security nor any court order," Anderson said. While content companies see obvious reasons to keep these sites offline, Mozilla sees it as a question of government censorship, and whether agreeing to these informal requests might somehow "threaten the open Internet," Anderson said.