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 configured, this preference controls whether to use keep-alive connections to the proxy server.
Possible values and their effects
Use keep-alive connections whenever possible. (Default)
Never use keep-alive connections.
First checked in
Has an effect in