The open-source Mozilla project said Thursday it won't comply with a U.S. Department of Homeland Security request to remove a Firefox add-on that helps redirect Web traffic for sites that have been seized by the government.

At issue is the Mafiaa Fire add-on, 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 a blog post Thursday.

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 pushed back, asking for proof that Mafiaa Fire is illegal, 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.

The DHS's ICE division could not immediately be reached for comment Thursday. Neither could Mafiaa Fire developers nor Mozilla's Anderson.

Robert McMillan covers computer security and general technology breaking news for The IDG News Service. Follow Robert on Twitter at @bobmcmillan. Robert's e-mail address is robert_mcmillan@idg.com