Halve your Firefox start-up time with the hackiest hack ever
Start Faster, which requires Firefox 4 and only works on Windows Vista and 7 at the moment, installs a service that speeds up the loading of Firefox's gorillaesque DLL libaries. The service runs with Administrator privileges, which allows Firefox to bypass the Windows prefetcher, improving start-up time by up to 100%. The one caveat is that you must launch Firefox using the new "Faster Firefox" shortcut that will be dumped on your desktop after installing the add-on.
If you want empirical evidence of the start-up speed-up, be sure to grab the About Startup add-on, which will show you your startup time if you visit about:startup. Both Start Firefox and About Startup are restartless, by the way!
Incidentally, if you don't like the idea of running an add-on that requires Administrator access, feel free to check through the source. We're fairly certain the add-on is safe, though.
Mozilla has toyed with the idea of using Windows services since 2009. They provide a great way of performing administrative tasks, such as updating Firefox in the background or defragmenting Firefox's databases. Start Faster is a proof of concept add-on that will hopefully be baked into a future version of Firefox.