My suggestion is to first establish your preferred profile. That is normally achievable by clicking help>moretroubleshootinginformation, scrolling down to Profiles, clicking about:profiles, and then selecting the displayed preferred profile. That gives you back the majority of what you seek. Your second request of merging mail files is more complex. A basic approach might be to export the messages from new profile and then import them after making the profile switch. There are a number of detail steps in that suggestion, but this might be enough to position you to complete the necessary steps.

Thank you David, you have spurred me on to delve further. This is what I concluded

Thunderbird stores it's user data folders in a *.default folder, called a PROFILE.

If you have installed a new version Ubuntu 22.04 64 bit Desktop, you may have 'lost' your Thunderbird data.

Shut down Thunderbird and assuming your have not changed the default paths for Thunderbird then:-

As of 6th May 2022 the *.default folder can be found here:-

/home/snap/thunderbird/common/.thunderbird/

You will also find the profiles.ini file at the above location.

Prior to 24th(??) April 2022 the *.default folder was at:- /home/.thunderbird/

When Thunderbird installs on the fresh 22.04 system it does not find an old profile so creates a new one. We will call this profile 'new.default'.

To recover your 'old.default' profile, find it at /home/.thunderbird/ and copy it to /home/snap/thunderbird/common/.thunderbird/

Then at the new location open the profiles.ini file, find the Path line and change the path from new.default to old.default. Save and start Thunderbird. Your profile name will actually look something like sa2y21xa.default, rather than 'old' and 'new'.

Editing the profiles.ini is actually a recipe for trouble.

Use the profile manager to create a new profile and point the folder path to the old profile.

Thank you Matt.

I have now managed to set up new profiles using the profile manager. I did this (for anyone following):-

Open Thunderbird Navigate Help>More Troubleshooting Information>about:profiles

You should have arrived at the About Profiles page which is the profile manager. Here you can create a new profile.

To get my precious old data into the newly created profile I opened the old *default profile and copied all the contents and pasted into the new profile, then set the new profile as default using the controls in the profile manager.

Matt suggested pointing the folder path to the old profile. This did not work for me so I ignored the path setting option and found my new profile in the current default path.

Matt's warning to not fiddle with the profiles.ini file should be heeded. I fiddled and managed to get sorted but frankly I was working blind not knowing the field parameters and the like. The profile manager gives you a safer working solution.