It doesn't seem to be possible to edit initial posts, so here's some additional info: I'm using v91.0.3. I deleted the Thunderbird profile folder, not the Thunderbird folder as I mentioned above. When I run Thunderbird, it doesn't use my copied profile (the profiles created during install have different names to my backup). Should I rename my backed up profile and give it the same name as the profile created during the install?

Deleting the existing Thunderbird folder in the Roaming folder and copying the entire Thunderbird folder from my backup solved the issue. However I have 6 months of missing emails. I'll post about this issue on a new thread.

Hi Either copy the entire contents of the Profile into the current Profile folder. Or, change Thunderbird to use the copied Profile instead of the current 'default' profile. You are using v91 - was the Thunderbird version you copied from the same version? It is not always possible to copy a profile over if the new installation is a different version of Thunderbird.

Copying the contents of my backed up profile folder didn't work, so that's why I had to copy the entire Thunderbird folder. Presumably this includes path information to the profile to use. The version of Thunderbird that I backed up from was either the same version or older. I discovered the reason emails were missing was because I was using an old backup instead of a newer backup of my profile folder. So this time I just deleted the profile folder and replaced it with the more recently backed up one.

When copying the profile, you do need to copy all the contents, not just selected folders, so yes, the settings and passwords etc are in the Profile. Profile being the folder name with a string of random characters that is inside Thunderbird\Profiles. However the path info is in profiles.ini, which lives in the Thunderbird installation folder, not the profile. Anyway, now you have discovered your missing messages so I guess all is well.  :-)