Thanks for the help everyone. I'm going to rephrase this entry to highlight the solution in case anyone else searches and ends up here.
I had to re-install all my data from a saved backup. I use thunderbird and wanted to keep my old saved emails. To create this backup I had copied the folder /home/[USERID]/ onto an external backup along with all its subfolders. To save emails, inside this folder is a subfolder .thunderbird/ , and inside that is another subfolder that has a name consisting of some random characters: let's call it [old_folder_name].default. So on my new system, I now had to take what was saved on the external hard drive (HD) in /home/[USERID]/.thunderbird/[old_folder_name].default/ and import it to my local machine.
Following advice I was given below, I launched thunderbird from scratch (after deleting the previous ~/.thunderbird folder to make sure I was starting clean) and then closed it again. That created a new folder inside ~/.thunderbird/ which I'll call [new_folder_name].default/ . I then copied the contents of [old_folder_name].default/ into [new_folder_name].default/ and relaunched thunderbird.
Normally, this should work. The reason I was having problems is that the external HD I was using wasn't giving me adequate permissions to manage these files, so I had to go to [new_folder_name].default/ and use
$ chmod -R 777 *
which might actually be a bit too permissive (I'm not sure if this is a security liability), but I just wanted to get something working.
To answer a question below: my external drive was using Ext4 formatting. To create the backup I had been doing this every once in a while:
$ rsync -rv [directory to be backed up] [directory to send to it]
sending the data from my local HD to the external media to back it up, and then vice-versa to put it back onto my local HD. I had assumed that that would preserve the same permissions (I kept the same username, although I did change the name of my PC in my new OS installation).
Can anyone suggest a way to rsync without running these sort of permission problems? (or, alternatively, another way to back things up where I can review the contents?) thanks again for your help.