...is the nglayout.initialpaint.delay one.

Everything else is fluff. Just do that single one. Why on earth it's not on as a default, I don't know, because it's worked on every installation of FF I've used, on every type of connection, from broadband to dialup to GigE LAN.

Not being familiar with the innards of the HTTP protocol specification, I cannot rule out that it's some oft-ignored but obscure requirement.