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. Using keep-alive connections improves performance. This preference determines how long keep-alive connections are kept alive.
Possible values and their effects
Amount of time in seconds to keep keep-alive connections alive. Default: 300 seconds.
First checked in
Has an effect in