If it makes you feel any better, I've had the same issue (Alma Linux, but also Flatpak) - https://support.mozilla.org/en-US/questions/1453148

I found on a blog post that an update went out when 128 was released which shouldn't have, so it's likely you and I installed it at that point, and then it was downgraded once flatpak was updated.

If you find the answer, let me know and feel free to add your "me too!" to my post to get it attention.

Thanks. I did that. I am now VERY worried. How does stuff like this happen? I know I am a mac user just trying to move to Linux, but these are the things that concern me about the future, I know I know, you shuold be more in control of your system, but not everyone understands what a 'profile' even is, and while people keep saying Linux (and Thunderbird) for that matter is user friendly and you don't need to be a tech head to use it, it doesn't seem the case sometimes. And worst of all, I have only recently moved from 20 years on Mac to Fedora, and not quite managed to set up backups yet, I REALLY hope I haven't totally frigged my email system as I am lost without it Hoping someone can allay my fears real soon

Start TB with the following command

"C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -p --allow-downgrade

Forgive me, but just pasting in random commands is not only ill-advised, for someone like me who stays as far away from Terminal as possible (unless essential), it can be dangerous. Could you at least add some context please? What will that command do? Will it 'fix' the entire issue? What else do I need to do?....

it will start Thunderbird but you will be allowed to use the profile which was affected by a new version of Thunderbirb. Just that. Nothing notorious.

Hardly understand what that means. I just want Thunderbird back working again. I only have one profile (i assume, never created any, just installed and set it up).

Obvious questions remain, IF I understand you properly.. for example, do I need to run with terminal command every time I start it?

As usual, volunteers on Ask Fedora forum come up with real solutions that Thunderbird people never do, with explanations I might add. One day someone will build something comparable to Thunderbird and I will be moving out and never coming back.

Much the same problem on pop_os, seemingly 3 years of emails and folder structure, address book lost.

But a search reveals .var/app/org.mozilla.Thunderbird/.thunderbird/v2hw2sr8.default-default/Mail/Local Folders/

contains the more recent folders...now, how do I associate them with the updated version?

I am no techie, but the two commands that solved it for me on Fedora were:

flatpak update --commit=2151b1e101f1266d135040c142f4c36a7e3373dc772e75a509a38efc39ded993 org.mozilla.Thunderbird

flatpak update --commit=cbb56c7054d2893acf77b5cdfd0adc0588adc090edb49a92d79ac6e8f7983ad4 org.mozilla.Thunderbird.Locale

In case you can somehow convert/translate whatever that does to your system.

Flatpak packaging was incorrect - this has been corrected.

If the newer version isn't causing a problem, perhaps let well enough alone, and stay on 128?

If that was to me, I don't understand. I had v128.0esr. In software update it's offering me, er, v128.0esr, the exact same version. So I am confused, do I allow it to 'update' since it appears to be the very same version? And do you know why it's offering an update when a new version isn't being offered?