Oh, great! I've just found out CCleaner also deleted completely my Mozilla Thunderbird configuration! Not only that but ALL MY ACCOUNTS AND MAIL TOO!
Great job, Pirifom! Well done! Clap, clap, clap, you deserve it!
Another member has pointed out that, rather than CCleaner being at fault here, it is a faulty Thunderbird update (52.2.0) that is the cause of this problem.
There are by now a few accounts of this problem with Thunderbird 52.2.0.
It seems to be random with some machines affected and others not.
I'm not sure about Firefox, Pale Moon, or Ice Dragon but as these are all based on the same Mozilla engine, and have also been updated this week, then they could all be giving the same fault.