I have experienced that Firefox 3 can be very slow during startup and when scrolling. It seams that these problems can be partially fixed.
Here are my favorite tweaks:
In the address bar type about:config and hit enter
Change the following:
network.dns.disableIPv6 false to true
network.http.max-connections 30 to 96
network.http.max-connections-per-server 15 to 32
network.http.max-persistent-connections-per-server 6 to 8
network.http.pipelining false to true
network.http.pipelining.maxrequests 4 to 8
network.http.proxy.pipelining false to true