From MozillaZine Knowledge Base
HTTP is the application-layer protocol that most web pages are transferred with. HTTP keep-alive connections can be re-used for multiple requests, as opposed to non-keep-alive connections, which are limited to one request. The preferences network.http.max-persistent-connections-per-server and network.http.max-persistent-connections-per-proxy define the maximum number of keep-alive connections that can be open at one time. This can cause a problem - if the keep-alive connections made are long-lived connections (large downloads), then short-lived connections won't be allowed to connect because the maximum number has already been reached. The solution is to allow the maximum number of keep-alive connections to be exceeded after a certain period of time. This preference controls that amount of time.
Possible values and their effects
Number of seconds keep-alive connections must wait before they are allowed to exceed the maximum. (Default: 10)
First checked in
Has an effect in