what is your mail providers retention policy? Do you have another device connected to that mail account? What are the retention policies on that device.

You have already said Thunderbird is set to delete nothing. So the deletions are occurring external to Thunderbird.

My email provider is Comcast and I just checked its email deletion policy and it WAS set for 90 days. I've now changed it so hopefully that will fix it. Is there any way I can retrieve my lost messages in Thunderbird?

Perhaps is the best I can say. If Thunderbird has not compacted the folders you might.

Install this add-on https://freeshell.de//~kaosmos/index-en.html#recDelMsg

Instructions to install https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Right click the folder use the undelete option. it will either work or not. There will be no some.

Restored mail goes into a sub folder of the original.

OK. I'll try this...I am comforted by seeing ALL of my emails dating way back in Comcast so, at the worst, I can go there to locate the older ones. thanks for your help

I can't find the exact problem I have listed. I have a trashcan in Thunderbird but there is nothing in it. however my Yahoo! with SWBT/ATT has the deleted messages. why are they not showing up in my trashcan? and how can I fix it.

WestCelt said

I can't find the exact problem I have listed.

Then the idea is to ask your own question.

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

It may be your subscriptions are wrong. Or it may be you are using POP instead of IMAP and therefore only our inbox downloads. But I am guessing the emails to HousTx about your problem probably look like spam to them. Hence the ask a new question.