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. If a proxy server is not configured, this preference controls whether to use keep alive connections on supporting servers.
Possible values and their effects
Use keep-alive connections whenever possible. (Default in all but Minimo)
Never use keep-alive connections. (Default in Minimo)
First checked in
Has an effect in