Please tell me what I should do to avoid that cancellation.

Well, if you want to keep the account that should be pretty obvious. It's stated in the message you received: "Log in to keep your account." https://support.mozilla.org/en-US/kb/mozilla-account-deletion-inactivity

This hasn't got anything to do with your actual email accounts set up in Thunderbird. Even if your Mozilla account expires you wouldn't notice anything in Thunderbird. There may be a greater impact for Firefox, e.g. if you sync your bookmarks to your Mozilla account.

If you don't know what a Mozilla account is all about, then I'm wondering why you did create one in the first place. It certainly didn't get created magically out of the blue.

I can't understand this messsage as I use daily several times the Mozilla products Thunderbird and Firebird. Please dont cancel my account as it would have catastrophic consequences. Please tell me what I should do to avoid that cancellation.

What the Mozilla Accounts are used for https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-account#w_what-services-can-i-use-with-a-mozilla-account as it is not used with the Thunderbird email client.

Mozilla Accounts was formerly called Firefox Accounts till Nov 2023 as it was changed to reflect on being used for multiple things besides say Firefox Sync.

This support.mozilla.org site uses Mozilla Accounts (since April 14, 2020) and if the email was about this same Mozilla account then by logging into your Mozilla Account here it should now be considered active and should not be deleted.