Here is my recommendation: install ImportExportTools on TB 24 and export the mail folders to mbox files (Export folder). Export address books to LDIF format in Address Book and export any local calendars. Then, uninstall TB 24, leaving the profile intact. Next, install TB 91, but don't run at the end. Open Profile Manager by opening a Run window, Winkey+R, type thunderbird.exe -p and create a new profile (so it doesn't try to open the old profile).

Add your accounts and install ImportExportTools NG. Import the previously exported folders to Local Folders (Import mbox file). Import the LDIF files (Tools/Import...) and calendars.

This avoids carrying over all the obsolete files from profiles created with old versions of TB. There's no significant difference in performance between 32 and 64 bit TB, but 32-bit is better if you send mail from 32-bit apps, such as scanners, image editors etc.

Thanks so much for your help sfhowes.

I've just finished exporting all my mail folders and address books. No calendars to worry about. I had to do this for both users (self and spouse). So unlike the other posters suggestions for upgrade methodology, your saying don't even bother installing all the other versions in order for the profiles to be migrated. Just uninstall the old version and install a new version using a new profile followed by importing everything.

Correct??

You made reference to importing to LOCAL FOLDERS! Since I have 7 different email accounts can I assume that the new NG Tools allow me to specify which account to import back into instead of LOCAL FOLDERS???

You also made reference to uninstalling the old version, do you see a potential issue with just installing to a new folder instead and leaving the existing version intact until up and running on he new version since I'd be creating new profiles for the new version??? In my thinking there may be an issue of duplicate registry settings between the two versions and conflicting somehow since two versions would be installed is why I ask.

One thing I've discovered and been searching on since you responded is that of exporting all my message filters. I just found where these are located. Looking at the contents it would appear that these are version independent Would you know if can simply copy these (msgFilterRules.dat) over to their respective mailbox locations???

I have also reset my master encrption password for my password manager to blank just to make sure that doesn't become an issue.

I also just found some info that indicates that the actual account settings for each email account are also stored in the pop3 mailbox folder. Will these be exported along with the actual mail and imported such that I won't have to recreate them all???

Tx again. Lots.

Some will tell you to create a new profile, some of those routinely make new profiles for each upgrade. I am still using the same profile I used with V2. My feeling is Thunderbird is broken if it can not update successfully.

Personally I would suggest you back up your existing profile and just upgrade over your existing profile. First to 86.5 and then to 78 allow it to auto update to 91. Since 68 there have been significant under the hood changes including Thunderbird having a built in control that reject downgrade use of profiles.

If it messes up you have a backup and can go back to V24 if you have to. Most folk will see some data loss, like passwords that have to be restated because they skip V68.5 https://bugzilla.mozilla.org/show_bug.cgi?id=1616495

Some might even see Thunderbird create a whole new profile for them if things get messy. But there are ways to recover use of almost all profiles post update. But it is an ugly bridge that is best left alone unless it is needed. https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

The bug I linked to contains a list of Thunderbird versions a profile should go through to get all the updates, but other than V68.5 to correct the passwords information, you will see the "watershed" versions are operating system or server specific. You appear to be posting from Windows 10. So the Linux GTK and macos changes are hardly relevant. Server certificate would already be not working for you if that was relevant

38.5.0 - SHA1 certificate stuff on AUS servers (if anyone is running anything less than this version they cannot use AUS) 48.5.0 - require gtk > 3.4, drop macos < 10.9, require sse2 52.9.1 - last version to support win xp/vista 60.9.1 - last version to support bz2 formatted MAR files

Ever hear the phrase "if it ain't broke don't fix it"? Thats why I'm still on 24.1.0

And that is why you are undertaking a lengthy consultation process asking for advice on something most folk do annually without an issue. I have had shocker upgrades, but never had to replace my profile.

You made reference to importing to LOCAL FOLDERS! Since I have 7 different email accounts can I assume that the new NG Tools allow me to specify which account to import back into instead of LOCAL FOLDERS???

No! You can move then around after import, but unless you are extremely careful with your account adding your POP mail accounts will become IMAP accounts (default on adding) and then you will largely be stuck with the mail stored in local folders.

Given internally billy@myuprodider.com is actually stored as settings for accountX where X is a number, your recreated accounts and folders for storage are unlikely to match what they used to be. You are basically undertaking a length recovery in anticipation of trouble. you have the backups, now try the update.

Note that if you have addons, disable or remove them. Most will not make the upgrade and will require removal and replacement with the new style (if the author updated them)

Note 2 All versions of Thunderbird in all languages and operating system can be downloaded from here http://ftp.mozilla.org/pub/thunderbird/releases/

The structure of the folders is Version, then operating system (including 32/64 bit where appropriate) then language.

Correct, I don't recommend installing intermediate versions. In theory, upgrades should migrate all data without issues, but in practice it often fails, as demonstrated by the annual cascade of complaints on this forum when a new version is released.

After the mail has been imported to Local Folders, it can be moved to POP account folders, but moving (uploading) to IMAP folders usually only works with small numbers at a time. IMAP folders will resync anyway on the first connection. Download headers only if those accounts have large sizes.

You can keep the old version, as long as the new one is installed to a different folder. I have a few versions installed, each using a separate profile, but only one is the default mail app.

Yes, the filter files msgFilterRules.dat can be copied between profiles and between accounts.

The account settings are stored in prefs.js, but I recommend adding the accounts manually in the new profile, especially if the server settings or authentication requirements have changed since TB 24.