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, the total number of HTTP keep-alive connections the application can make to each site is limited by this preference. If more connections are needed, they are queued until a connection "slot" is available.
Possible values and their effects
This preference takes values between 1 and 255 inclusive, directly corresponding to the maximum number of HTTP keep-alive connections the application can have open at once to a single server. (Default: 2. Firefox 3: 6)
If you are not using a proxy and experience problems not being able to download multiple files, you can raise this value. It is, however, considered poor etiquette to make too many connections to a server and may lead to you being banned from that server. Anything above 10 is excessive.
First checked in
Has an effect in