I spent hours scouring the net and finally found how to get Thunderbird version 115.12.2 on Ubuntu 24.04 to connect to my Gmail account.
The reference is from:
by: https://superuser.com/users/1109344/feldmarshall
Essentially you have to use the Thunderbird Config Editor to change the "general.useragent.compatMode.firefox" property to true.
The config editor can be found under Edit-Preferences-Advanced-General-Config Editor