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).
To clean temporary folders, check and delete all files [you can, some may be in use]from these:
C:\Documents and Settings\*user*\Local Settings\History
C:\Documents and Settings\*user*\Local Settings\Temporary Internet Files
C:\Documents And Settings\*user*\Local Settings\Temp
C:\Documents And Settings\*user*\Recent
On Windows Vista & 7:
%USERPROFILE%\AppData\Local\Microsoft\Windows\Temporary Internet Files
To delete Internet Temporary Folders, in the Control Panel., open Internet Options. In the Browsing History section, press the Delete… button, check all the options and press Delete.
Users report a significant improvement after doing this, even those who use Internet Explorer once in a while.
A fix to this bug should become available with Firefox 3.5.1, expected in a few weeks.
Edit: Thanks to Dan in the comments I’ve updated the post with the correct Windows Vista/7 paths for temporary folders.