The latest versions of CCleaner (say 5+) corrupt my Thunderbird files. In particular my saved searches in Thunderbird loose all their settings and become useless. CCleaner did not used to do this. I am using Thunderbird v 52+. I have found that when I stop having CCleaner do ANYTHING to my Thunderbird files, the problem went away. It is clearly due to CCleaner. It's not a show stopper, but I would rather CCleaner clean up internet related data created by Thunderbird.