Why Firefox 3.5 Startup So Slow?

Let's see what mozillalinks.org says:

If you find yourself with very long startup times after upgrading to Firefox 3.5 (from say 10 seconds to the order of minutes), you may be experimenting a bug due to a change in how Firefox 3.5 gets the randomness it needs for security purposes on Windows.

The procedure involves scanning some temporary folders looking for bits normally added by OS and other applications operations. Firefox 3.5 looks for more files and deeper (more subfolders) for increased randomness, but it has led to unexpected results for users with too many temporary folders or files resulting in slow startups.

Try builds are still being generated with fixes to this bug, but users report a noticeable improvement after deleting their temporary folders and Internet Temporary Files (generated by Internet Explorer). Read more .....

And from support.mozilla.com:

It has been proven there is definitely something wrong with the startup of Firefox 3.5 as it is too slow. Bugs have been filed and a fix is in the works. I'm also trying to get a test build with the problem backed out to see if it loads fast for you guys again. So please no more threads about the issue and stay tuned to this thread for updates on the issue.

Currently, the workaround that helps a lot of people is to clear your Internet Explorer temporary internet files & Temp folder files. It may or not help you but its worth a shot. Read more .....

That's why.