Userscripts.org

I am currently doing all my testing using Firefox 11. With the current configuration in "about:config" which you can access by typing into your url bar, changing the settings and then restarting your browser.

browser.panorama.animate_zoom false
browser.safebrowsing.enabled false (don't change this if u frequent malicious sites.)
browser.safebrowsing.malware.enabled false (read above)
browser.sessionhistory.max_entries 1 (will disable the back button,limits cache)
browser.sessionhistory.max_total_viewers 1 (read above)
browser.sessionstore.enabled false (restores your previous session, waste of cache)
browser.sessionstore.interval 6000000 (interval to store, waste of memory)
browser.sessionstore.max_tabs_undo 0 (obvious)
browser.sessionstore.max_windows_undo 0 (obvious)
browser.sessionstore.resume_from_crash false (i think we can manage this ourselves)
browser.tabs.animate false (trade off, graphics for performance)
browser.tabs.autoHide true (i like this, maximizes viewing area when only 1 tab open)
browser.urlbar.autocomplete.enabled false (annoyance)
browser.urlbar.maxRichResults false (trade off)
browser.urlbar.trimURLs false (i like to know if i'm on a secure web page)
dom.ipc.plugins.enabled false (firefox trying to act like chrome, blah!)
layout.css.report_errors false (these things are dime a dozen)
network.http.pipelining true (just do it! lol)
network.http.pipelining.maxrequests 6
network.http.pipelining.ssl true
network.http.proxy.pipelining true
network.prefetch-next false (this is a biggy)

that's it, you can google each setting if you want to get a headache.

To enable profile manager for those of you who like using multiple profiles.

Right click on your shortcut to firefox.

In the text-box labled target, change the line as such.

My default setting: "C:\Program Files\Mozilla Firefox\firefox.exe"
To enable profile manager: "C:\Program Files\Mozilla Firefox\firefox.exe" -no-remote -profilemanager

With the current settings and assuming you haven't added tons of add-ons, you should be running around with 100 Mb/s per profile with an occasional spike of up to about 15% cpu usage, on average, it is more like 8% running tools, bot, and kocattack. Enjoy!