A while back, I mentioned that Firefox profiles will be migrated over from the old "Phoenix" directory to a "Firefox" directory. Ben Goodger (Firefox lead engineer) postponed the migration from last Friday to yesterday, and the latest nightly builds now include this change (see bug 203077).
If you are using an official nightly (or an unofficial nightly - see the MozillaZine Firefox Builds forum), you will be prompted with this screen when starting the new build if you have more than 1 profile:
Select the profile you want, and your settings and preferences will be imported:
At this time, it appears that the saved form information is not imported. Can anyone verify this?