-------------Thunderbird installing latest version as Flatpak to work with old TB profile:
A)
UbuntuZilla repository package had similar version like Debian old-stable:
echo -e "\ndeb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main" | sudo tee -a /etc/apt/sources.list > /dev/null && sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 2667CA5C && sudo apt-get update && sudo apt-get install thunderbird-mozilla-build
# Commands according to https://sourceforge.net/p/ubuntuzilla/wiki/Main_Page/#installation
It installed:
thunderbird-mozilla-build/stable,now 102.14.0-0ubuntu1 amd64
old-stable is:
thunderbird/oldstable-security,now 1:102.14.0-1~deb11u1 amd64
It created empty new profile apparently, but after i removed the package, i was able to continue using old profile:
sudo apt-get remove thunderbird-mozilla-build

B)
Then I have tried: flatpak search thunderbird # has v. 115.1.0
flatpak install org.mozilla.Thunderbird # App launcher was added to the menu
It launches with a new profile. Menu Help / Troubleshooting information / Profiles / about:profiles shows:
Root Directory /home/user/.thunderbird/abcdef.default-release
Local Directory /home/user/.var/app/org.mozilla.Thunderbird/cache/thunderbird/abcdef.default-release

Reading https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data I have tried to symlink old profile directory to new location:
ln -s /home/user/.thunderbird/abcdef.default-release /home/user/.var/app/org.mozilla.Thunderbird/cache/thunderbird/
But it have NOT detected it inside mentioned about:profiles page. Also appending "-profilemanager" to Exec line in file /home/user/.local/share/applications/org.mozilla.Thunderbird.desktop has no effect:
Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=thunderbird --file-forwarding org.mozilla.Thunderbird -profilemanager @@u %u @@

WHAT HELPED was to give filesystem access permission by running: sudo flatpak override org.mozilla.Thunderbird --filesystem=host
Then i was able to remove empty default profile from mentioned Help / Troubleshooting information / Profiles / about:profiles.
And set as default my main old Thunderbird profile which appeared on that page.
And it works.

Old TB now refusing to launch that profile since it was likely marked or converted into new one. So i hope i did a good and picked not so bad way to run newer TB. If you know other, more simple way to run it, please share. Thank you