After having bumped into the profile switcher extension, I jumped through the hoops to move part of my main profile into a secondary profile.
The upcoming Firefox 3.5 has a Porn Private Browsing mode, but I find it easier just to use another profile. This way I can choose to maintain cookies and sessions, and I can also have separate bookmarks.
Two profiles running on the same user seems to share the same memory, which is a serious advantage over multiple users with separate instances.
It was a bit annoying splitting my profiles, because it appears that add-ons have some hardcoded paths. Thanks to the magic of a real filesystem, I created a symlink and was done with it. I don't feel like screwing around with reinstalling addons. kthxbai.
Also, I remember there used to be profile passwords. What ever happened to those? I know it's not real security, but it's still nice to have.