At least on Windows they are both multi-threaded. If you look in Task Manager then firefox.exe will have multiple threads (32 on my machine) and each chrome.exe

At least on Windows they are both multi-threaded. If you look in Task Manager then firefox.exe will have multiple threads (32 on my machine) and each chrome.exe process will have multiple threads (anywhere between 3 and 21 on the 15 tabs I have open).