From MozillaZine Knowledge Base
HTTP is the application-layer protocol that most web pages are transferred with. The total number of HTTP connections the application can make to a single server 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 connections Mozilla can have open at once. (Minimo default: 2. All others default: 8. Firefox 3: 15)
If you experience problems not being able to download multiple files from a site, 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.
First checked in
Has an effect in