After installing Portable Thunderbird, I followed the instructions under Copying Your Local Thunderbird Settings (https://portableapps.com/support/thunderbird_portable#installing) to give Portable TB the environment I had been using with non-Portable TB. Of course, I copied my non-Portable prefs.js file along with everything else.
I notice that in the prefs.js file there are references to C:\, e.g. --
user_pref("mail.root.pop3", "C:\\Documents and Settings\\Rjd\\Application Data\\Thunderbird\\Profiles\\32umcxms.default\\Mail");
What is the canonical procedure for clearing rubble out of prefs.js?