On Thunderbird I keep seeing a message that asks me to purge deleted messages. [1] Does this mean that if I check this I will lose all messages in Trash, which I do not want to do. [2] If yes how can I stop the purging message?

You are probably seeing the prompt which is about "compact".

[1] no [2] you can use the checkbox in the compact prompt so that it stops asking you - the dialog prompt is just a courtesy, and disabling it doesn't break anything.

https://support.mozilla.org/en-US/kb/compacting-folders talks about compact

In Version 78*: Menu app icon > Options > General Scroll down to 'Network & Disc Space' section under 'Disk Space'

Do you have this selected: 'compact all folders when it will save over xxmb' ? If yes, uncheck that option restart Thunderbird.

I suggest you also compact your folders to clean them up. This is particulary important for : Inbox, Junk, Drafts as these folders get the most activity for moving anfd deleting emails.

Right click on folder in Folder Pane and select 'Compact'.

> 'compact all folders when it will save over xxmb' ? If yes, uncheck that option restart Thunderbird.

This is a bad idea unless you are going to do compacts forever, frequently, manually on your own.

Version 91 comes out in several weeks, in which the language of the compact dialog has been greatly improved. When you install version 91, if compact causes problems or is unclear, please create a new support topic.