Firefox limits the number of simultaneous connections to a single server. If more than 6 threads are needed, increase network.http.max-persistent-connections-per-server and network.http.max-persistent-connections-per-proxy in about:config .
So you can allow users set default number of threads more than 5 for any download type.
Multithreaded Download Manager developer said it is possible to accept more threads for a download task:
So you can allow users set default number of threads more than 5 for any download type.