Import TB Profile from Apt TB78 to FlatpakTB91

I want to change from apt installed Thunderbird 78 to flatpak install Thunderbird 91. What is the most efficient way to get my Thunderbird 78 profile to become my default flatpak Thunderbird 91 profile?

Using: Ubuntu 20.04

Profile Locations:

• TB78 /home/user/.thunderbird
• TB91 /home/user/.var/app/org.mozilla.Thunderbird

After trying the solution below, I get:

This is the correct profile directory, but TB91 ignores TB78 files & settings (no import). Additionally, I know that after successful import into TB91 that TB78 should no longer be able to open the profile, so I tested and opened TB78, works perfectly.

Update:

I caught my error, instead of selecting: /home/user/.thunderbird
I should have selected: /home/user/.thunderbird/frefref.defautd-user

Turns out I didn't make a mistake.. TB91 will not allow me to import the TB78 profile:

My TB78 profile is in that folder but it is not being shown as a choice by TB91

There is no need to import profile. You can just use the same profile from thunderbird 78. For this you have to open thunderbird 91. Open help from the menu and then select "more troubleshooting information". In the 'Application Basics", click "about:profile".

A window open up which lists the profile already existing in the PC. Select the profile which you use for Thunderbird 78 and make it default by clicking the button underneath the profile name. Close Thunderbird and open.

if your profile is not listed when you click "About:profile", simply create a new profile and select the location of thunderbrd 78 profile by clicking on "choose folder" button.

Either way you should be able to see thunderbird 91 with your thunderbird 78 profile.