I tried just backing up my profile and then replacing everything in the new profile folder with everything from the old profile folder, but that didn't work. Finally (and for future reference) what did end up working was the advice given here. I copied my old profile into Documents & Settings/user/Application Data/Thunderbird/Profiles, then opened up the profiles.ini file and redirected it to look at the name of my old profile. Then I was able to delete the new profile. Et voila! It's all there :) Whew!