The 20-line patch to Firefox 4 that makes startup on Windows up to 2x as fast and the stupid one-liner that does the same on Linux both grew into a full fledged preloading solution working on all our supported platforms. This involved major changes to how we initialize Firefox, and a few glitches with our leak detector, but this time it should stay for good (it had been backed out twice already).
Users shouldn’t notice any change until after they reboot after upgrading to the latest nightly. It is possible to watch how things evolve with the about:startup extension.
These cold startup improvements will be available in Firefox 7.