I have mostly the default Firefox settings, which are:
max connections: 24
max connections per server: 8
max persistent connections per proxy: 4
max persistent connections per server: 2
However, I also have manually set the following, based on the recommendations in the thread
Firefox Tuning - MozillaZine Forums:
network.http.pipelining: true
network.http.proxy.pipelining: true
network.http.pipelining.maxrequests: 8
Since I'm using a tabbed browser, I typically open 10-12 tabs at a time, and the last couple of tabs trip over into the Access Forbidden page.
Hope this helps, let me know if you need anything else.