After some research. Removing the start page seems to be the solution. However this only started after the update.

https://support.mozilla.org/en-US/questions/1092339

After some research. Removing the start page seems to be the solution. However this only started after the update. https://support.mozilla.org/en-US/questions/1092339

https://www.reddit.com/r/Thunderbird/comments/1jtllao/comment/mlvpwwi/ hints that it is being caused by the conversations addon. This is confirmed by at least one other person.

What version of Thunderbird are you using?

I can also confirm that reddit hint seems valid for version 128.9.1esr (64-bit), same as reporter noted in title.

Disabling the conversations add-on and restarting Thunderbird will effectively allow a user to remove the 'Donate' welcome page by clicking an email message, allowing the e-mail message to take up the entire Message Pane.

Thunderbird Version: 128.9.1esr (64-bit) Thunderbird Conversations Version: 4.2.5

The issue here seems to be that Welcome Page takes up half the reading pane and cannot be removed when using the above versions.

I contacted the awesome author of https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/ - this is now fixed in a new version of the addon. This should only have been affecting users of Thunderbird 128.