https://abcnews.go.com/amp/Politics/firewall-counsel-protect-government-secrets-russian-election-meddling/story?id=57068964&__twitter_impression=true
What is a "firewall counsel"?
Generally, a chinese firewall. A organizational structure that has a "firewall" or "filter" that prevents certain people from seeing certain things.
In this case, Mueller is trying to hide evidence from the defendants, so the judge is allowing a firewall.
In trials, the usual firewall is the judge, personally, who reviews ALL the evidence, and decides what part the defense is entitled to see. CIPA is a form of firewall when defendants are charged, and parts of the case involve classified material.
Zaid explained that if Concord wants to show some information to outsiders -- say, a subject matter expert -- the company's U.S.-based lawyers would have to seek permission from the firewall counsel first. The firewall, in theory, protects both U.S. government secrets and the defense strategy.
Perkins Coie is one. Use of attorneys to shield conduct by criminals.