Were you using firewall software on your computer? If not then maybe you were letting in a bunch of traffic that the router's internal firewall has now put a stop to. Can't think of another reason why a router would speed things up really.
I think a bit more information is required. It sounds to me like something is hogging your system resources, which is probably why folders are slow and FireFox won't open. Are all your programs running slower than usual? Is it specific applications?