Comment 48 for bug 927515

For someone looking for a workaround, then the 100% CPU usage could apparently also be a result of TB reopening/reindexing databases too often. According to these links:

   https://bugzilla.mozilla.org/show_bug.cgi?id=794401
   http://rainbow.chard.org/2013/02/19/thunderbird-high-cpu/

it appears that "Technically ... [Thunderbird] check open DBs every minute, and if they have been open more than 3 seconds and there are no other references to them, they are closed. The '3 seconds' has now been changed to the original intent of 5 minutes."

On my system, mail.db_idle.limit was set to 300000, which might have been the intended value... But I've increased it to 30000000 as others have suggested, which seems to help -- I have many really really large IMAP folders, so indexing is a slow process that can last minutes.

Whether it has negative impacts elsewhere remains to be seen. But it is an easy tweak and fast to test