Using the same strategy I used for Firefox and Thunderbird in Vista, I managed to create a Firefox setup in Win 7 that uses my ONE common profile, stored on my XP drive. No problems.
However, when I try the same thing in Thunderbird, I get the dreaded message that the profile is in use. I still have the link to a Mozilla page that Tony pointed me to when I did Vista a year ago. But, I don't have any parent.lock file so I can't figure out what's blocking the execution. BTW I'm in Win 7 as I type, in the same Fx profile I use for XP and Vista.
I'll continue to look in Mozilla stuff, but if anyone has a hint, I'd appreciate it. (I already have Win 7 set to show ALL files in Windows Explorer and hide nothing.)