It disables pipelining by default... made it much slower for me too, but I followed these steps I found on another forum, and it's speeding along faster than ever now:
Open Firefox (Fx), type about:config into the address bar, and hit return.
Locate the following entries:
network.http.pipelining
network.http.proxy.pipelining
network.http.pipelining.maxrequests
Double click each entry, and alter them as below:
Set network.http.pipelining to true. As set initially, Fx loads only one page at a time.
Enabling pipelining causes it to load several at once, which makes switching from page to page
much faster.
Set network.http.proxy.pipelining to true.
Set network.http.pipelining.maxrequests to 30. This causes Fx to request 30 pages at once. (I
changed mine to 100 and it works perfectly, but I've got lots of memory. I wouldn't go over
thirty unless you have at least 512 MB of RAM.)
Right-click anywhere on the page and select New - Integer. Name it nglayout.initialpaint.delay
and set its value to 0. This changes the amount of time that Fx waits to act on information.
Your pages should load noticeably faster. Enjoy!