RE: All I want out of FF3...
At the moment I dual boot between Windows and Ubuntu, in addition to the operating system partitions Ive got a small fat32 one where I moved my Firefox and Thunderbird profiles. Both OS' installs look to the profile on the shared partition for the bookmarks, cache, mail etc, that way I can access them whatever I boot in. It didnt require any plugins, just the use of the profile manager (e.g. firefox.exe -P) to tell the app where to look.
Beyond the need for a small home server/always on PC, is there anything stopping you doing the same, but using a network share instead of a partition?
Though its orientated for sharing a partition for dual booting, there's an guide here: http://ubuntuforums.org/showthread.php?t=203524
If you specifically just want to be able to sync profiles without messing about with network drives, this might be worth a look: http://lifehacker.com/software/hack-attack/sync-your-firefox-extensions-and-profiles-across-computers-272113.php