The error is almost always a missing zone file in the single-player campaign. The simplest fix:
Verify game files via Steam → relaunch → if persists, check antivirus quarantine → reinstall.
For modded clients, consult that mod’s specific documentation, as zone naming conventions may differ. Modern Warfare 3 Could Not Find Zone Sp Intro Ff
Temporarily disable AV, then verify files again.
Add the entire MW3 folder to AV exclusions. The error is almost always a missing zone
MW3 is a 32-bit application. If you have more than 4GB of RAM, it usually fine. But if you forced the game to use high-resolution textures (via config.cfg edits), the .ff file might fail to load into memory. Temporarily disable AV, then verify files again
Plutonium reroutes the zone directory. Ensure your plutonium-storage folder has the correct game_export files. Run the plutonium.exe as admin and use the -fixfiles launch argument.