how about you let this essential house keeping task actually complete. It is not compress either, it is compact and they have different meanings. If you are using a non English version that is using a localized word that means compress rather than compact. Could you please advise the language involved to we can ask the localizer to use correct terminology.

Compress to my mind implies something will be happening to make your mail smaller. Compacting does attempt to reduce the file size used to store your mail, but it also acts a a final purge of mail you have deleted since the last compact. That means that your deleted mail is available to be undeleted. It is still there until a compact occurs.

Thanks a lot for your quick response. I use a German UI and I believe compress is meant reducing space (see attachment). Is it possible to disable this question or to specify a size when it shall be displayed?

Options > Advanced > Network and disk space has options for both disabling it and modifying the saved space.

Just keep in mind that the "saved space" is currently occupied by mail you think you have deleted. To understand just how much of that there is, install the RECOVER DELETED MESSAGES addon and apply it to your inbox folder.

So while it might be convenient to have it only function when there is say 500MB of deleted messages in your file. That might be the last 5 years worth or longer. Given an average of 200kb per message, the numbers add up very fast and represent a security and privacy risk that is to me unacceptable for more than a few months at a time.

There are also performance implications with disabling compact. (German has translated the Thunderbird term "compact" to compress)