Thunderbird "stops responding" at least one a day. I can't say that I've seen a pattern as to when it happens. I'm at a loss to explain it and have not been able to find a reference to this problem anywhere else. The only way out have have at this point is to "force quit" the app. I'm running OS X 15.5.5 on a MacBook Pro with Thunderbird 220.127.116.11.