The FIREWALL Act prohibits federal funding from going to individuals and entities that purchase fiber-optic cable from companies owned or controlled by countries of concern, primarily targeting foreign adversaries like China and Russia. The restriction applies to federal funds administered through the Federal Communications Commission and the National Telecommunications and Information Administration. The prohibition takes effect 90 days after the bill becomes law and applies to any fiber-optic cable purchases made on or after that date, including both solid-core and hollow-core fiber varieties. The bill uses the existing definition of "covered nations" from federal law to determine which countries are considered countries of concern. This legislation is intended to protect U.S. telecommunications infrastructure from potential security risks by ensuring federal funding does not support the purchase of critical communications equipment from adversarial nations.