Why not just let the software manage your upgrades? Help > about will identify the next update you need and download it. Unless you have done something to break the update process, that is the preferred method.

There has been many updates since v78. All of those major updates - v91, then v102, then v115, and so - have had various changes. Some of those changes involved format changes for files such as those used by address book.

It might seem a bit long winded, but it means you can get each stage done so it's in a state which the next update is ready to use. I would advise that you create a backup of your Thunderbird profile. if you use Windows OS and default location for storage then it will be here: C://Users/username/Appdata/Roaming/Thunderbird You must exit Thunderbird before you create the backup. See image below which shows you how to locate the 'Thunderbird' folder.

You would need to get a copy of that Thunderbird folder and paste it somewhere appropriate to act as a backup eg: on D: Drive or create a folder on Desktop and call it 'TB-Backup' - in that folder create another folder which has the version number as folder name and put the copied 'Thunderbird' folder into that eg: 'TB-v78' folder.

After each update if everything is working ok - I would suggest you perform a backup again. In 'TB-Backup' create a new folder with the new version number eg: 'TB-v91' and put another copy of 'Thunderbird' folder into it. Repeat this until you get up to date. Assuming all is working ok. The last version will be whatever is current eg: 128.3.0 If all still ok create the last backup - then you can delete all the other backups and finally empty the Computer Recycle Bin.

When it comes to installing old versions, I'll suggest specific versions because a few gremlins were being ironed out when each new major version was first released, so these versions were the first ones which were released as an upgrade. Remember to uninstall just the Thunderbird program before installing new version.

Version 91.1.2

• http://ftp.mozilla.org/pub/thunderbird/releases/91.1.2/

Version 102.2.0

• http://ftp.mozilla.org/pub/thunderbird/releases/102.2.0/

Version 115.2.3

• http://ftp.mozilla.org/pub/thunderbird/releases/115.2.3/

When you use those links, you then need to click on appropriate OS eg: win64 Then choose your prefered language eg: en-GB for English British Finally click on the exe file link eg: Thunderbird Setup 91.1.2.exe to download.

Matt's advise is equally valid.

So you have two methods. But do create a backup - if something goes wrong at any stage, you will have a backup of the last working profile meaning if you lost eg: an address book then it can be recovered.