alerts missing about mailbox-alert HOT 1 OPEN

The preferences are stored in the file prefs.js in the profile directory on your hard drive. If you still have the old profile directory on your hard drive you can try to manually copy all the lines that have "mailboxalert" from the prefs.js file of your old profile to the prefs.js file of your new profile.

The exact location differs per operating system, see https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data and select your OS on the right.

Note that it is important to make sure thunderbird is not running while you do this, as it would override your changes when it exits. It's also of course wise to make a backup copy before you start editing files like this.

from mailbox-alert.