This is a major improvement for the start-up time of the Firefox. The “Preload Firefox libraries at startup” feature will make the startup two times faster on Windows and Linux platforms. And, this feature is now live in nightly builds available here. Firefox 7 will be shipped with this feature integrated.
To test this feature, install the latest nightly build of Firefox and reboot the system. You can also install this about:startup add-on to monitor start-up performance of the Firefox. If you are not comfortable with the nightly build, wait for Firefox 7 to see these improvements.
Developer Mike Hommey explains preloading, "There are 11 .so files in the Firefox directory, some of which are really small, and a few being bigger […] Even when the filesystem is fragmented, there are good chances that the files are stored in chunks of significant size, and that these chunks are more or less ordered on disk. In our case, where the files are read in great part during startup (more than 80%), it’s obviously going to be much faster to read them entirely than to randomly read small chunks from them. Which is why it works so well.”. Mike has described preloading in detail on his blog.