20-line Firefox fix will half start-up time, hopefully coming to nightly builds tonight
To be honest, it's a little odd that preloading DLLs hadn't been tried before. Prior to this fix, the XUL DLL was slowly loaded in 32KB bits, which really ground on slower-seeking hard drives. With a little trickery, the patch submitter managed to get Windows to load the DLL in juicier, faster 2MB chunks. The fix should improve load times on every computer, but it will definitely benefit slower computers more.
As far as we can tell, the fix hasn't landed in nightly builds yet. It should be soon, though: Mozilla's VP of Engineering, Mike Shaver, wants to "land this bad boy for Monday's nightlies" -- so keep an eye on Mozilla's public FTP dump tonight.
Incidentally, if you're not using the beta 10 nightly build yet, you should give it a go!