Firefox 4 on Windows: One of the longest-running complaints among the anti-Firefox crowd is that it's slow. One Mozilla developer has set out to fix part of this problem by making Firefox start up twice as fast in Windows with a simple add-on.
In Windows, Firefox calls on a few different things at the OS level that tend to slow down its startup, namely some hefty DLL libraries. Mozilla developer Taras Glek says there are quite a few optimisations Mozilla has yet to include, like bypassing the Windows prefetcher to account for this slowness, so he built an add-on that included them for the time being.
Just install the add-on and use the shortcut it creates on your desktop to start up Firefox. You should see a pretty noticeable increase in how quickly Firefox's window shows up and how fast it calls on your old tabs (I certainly did). If you're skeptical, you can use the About Startup add-on to benchmark it yourself. Hit the link for more info, or just download it and test it out yourself.
Start Faster is a free download for Firefox 4 on Windows only.