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 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 integer preference takes values between 1 and 65535 inclusive, directly corresponding to the maximum number of HTTP connections Mozilla can have open at once. The default value is 24 for all applications except Minimo, whose default is 2.
Users on slower connections may want to reduce this number to help prevent HTTP connection timeouts. Users on faster connections may want to increase it, but the default value is usually fine for normal usage.
First checked in
Has an effect in