What you see as a minus sign I suspect is a partially rendered triangle. The later is intended to indicate that the number which follows is a count which totals the messages of all the subsidiary folders.

Try updating to Thunderbird 45.1 to see if the icon improves. Or try starting Thunderbird in safe mode

https://support.mozilla.org/en-US/kb/safe-mode

Thanks, Wayne

This question has been locked because the original author has deleted their account. While you can no longer post new replies, the existing content remains available for reference.