I was plagued with this problem on my XP older computer. I tried most of the above remedies and the best time I ever got was about 17 seconds on a cold boot. I was told that the reason IE loads faster is because it is partially loaded in the OS on boot up but FF doesn't have that benefit. I now have a new computer and immediately installed FF when I got it. In the beginning with all my add-ons installed, FF loaded in 4 to 6 seconds on a cold boot. After 5 months it is now creeping up to about10 seconds. I periodically delete the cache and don't let it go higher than 100Mb. As a off the top of my head possible remedy, maybe you could try saving your bookmarks and uninstalling FF, then re-installing it. A new install will probably have a nice loading time.
My Chrome still loads in 4 seconds, so they must have a secret.