I have found a few Thunderbird-related leftovers. I use Win7x64. I have been using Thunderbird since the previous version (91.5.0 I suppose). I've been using two extensions: Dark Reader and Thunderbird Conversations.
LEFTOVERS
Files:
1. AppData\Local\Thunderbird
2. AppData\Roaming\Thunderbird
Registry:
1. HKEY_CURRENT_USER\Software\Thunderbird
2. HKEY_USERS\***pc-user-personal-id***-Classes\Software\Thunderbird
***pc-user-personal-id*** - it's a random string which is different for each Windows user profile
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I've just noticed that there is still a leftover at "AppData\Roaming\Thunderbird". In my case there's a file "InstallTime20220305171341" at location: "AppData\Roaming\Thunderbird\Crash Reports".