Hello,
I'm surprised this article includes no information on how to migrate from Portable Thunderbird to the regular version, although there must be enough users out there who would like to do just this after trying it out safely through the Portable version.
https://portableapps.com/support/thunderbird_portable
Cheers,
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To copy back, reverse the process. Note that it's best to have run a local version at least once so it sets up the local profile directories for you to copy into.
Sometimes, the impossible can become possible, if you're awesome!