1. Exemplary implementation of user accounts. I have a separate user account for my private Google account (Gmail, Calendar etc), a second for Facebook, a third for work (separate Twitter and Google accounts) and a third for web development. I know Firefox has a Profile manager and that it is possible to use -no-remote to run several profiles at once. But: it does not work very well at all. Not even with the https://addons.mozilla.org/en-US/firefox/addon/profileswitch... addon.
2. Incognito window can run in parallel with the "normal" browsing session. This is lovely when I want to see how a website looks in a clean browser. In Firefox the private session replaces the normal session until i chose to go back to "unprivate" again.