The maximum number of connections is one of the hidden options that you can change through about:config, and there's also a Firefox addon which can set different levels. It's generally considered to be a bit antisocial to set the value too high, because websites have limits on the total number of connections they can support, and they don't want a comparatively small number of clients saturating their capacity. There was a bit of a fuss about this a few years ago, and I think that since then, FF has maintained a lowish default value.