Here's the bug for this slowness:
https://bugzilla.mozilla.org/show_bug.cgi?id=501605
At least one problem is a change in the way FF gets randomness. It scans a bunch of temporary files from several sources at startup and reads selected files (about 20) from the list of files it builds. Reading 20 files isn't a big deal, but scanning very large and deep directory trees can.
People who use the IETab addon will be dumping lots of temporary files into the Internet Temporary Files store. (And if you care about privacy, it will also silently leave lots of history there that Firefox doesn't know about and hence can't clean up for you.)
On Red Hat Linux and derivatives (RHEL, Fedora, CentOS), there's a nice utility called tmpwatch in the default setup that scans your /tmp directory each night (via the system crontab) and deletes all files that have not been accessed in the last 7 days. (Note that's access, not modification. So you don't have to leave the file unwritten to see it deleted. Opening an ancient file for reading will update the access time and prevent it from going away.) Anyone know of a similar utility for Windows?