The setting within Thunderbird that could impact that are found at <Accounts Settings | Synchronization & Storage | Disk Space>. There you can set it to delete messages over a certain number or a certain age. You can also set this per folder. That is found by a right-click on the folder name, choose "Properties" from the context menu, then the "Retention Policy" tab.

Keep in mind that if you have other devices, like phones or tablets, that access this account, they could be deleting things as well. Also, your email provider might also have their own retention policies.

In 'Disk Space' I have set it to "synchronize the most recent 30 days", and set "don't delete any message". I guess I will have to wait several days to see if the older messages are still kept. tnx for your help. Al