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The issue is resolved, I just don't know how to mark this thread as such. The problem was the registry cleaner removing MRU files (Most Recently Used) Unchecking this option solved the problem

Installed winnap2 and got a whole list of cleaning options showing up which I didn't have before. Some of these choices have an asterisk *at the end. Can anyone tell me what this means?

All e-mail related defaults are set to Thunderbird.

You are right, the problem has returned. Therefore, I suspect one of the Windows system selections. I guess, the best way is to test is to deselect all and reenable them one by one. Any other suggestions would be most welcome.

I believe that it rebuilt ALL the rules (they were quite old and possibly damaged). I did have a problem with Thunderbird and CCleaner long ago which I posted on Mozillazine. One of the moderators helped me to narrow down the problem to Ccleaner and I cleared all rules for TB. This helped but it's been years since I had a clean install, only updates over updates of numerous versions.

Installed winapp2 and this resolved the Thunderbird default message problem.

After reading up on winapp2, I installed it. It's impressive. But I still don't know why the message comes up. Oh well, I wil post the issue in the Mozilla forum. Again thanks for the replies.

Thanks for all the responses. @hazelnut, nothing in Advanced is checked. @mta, It's Windows 7 Pro 32 bit SP1, - I do not run the registry cleaner of CCleaner, or any others when this message comes up. It's actually called the System integration message, I just took a screenshot and included it. It is not like the Mozilla Firefox "default" message. I see "winapp2.ini" mentioned in several places but have no clue as to what it is.

In the past 5 or so versions of CCleaner (and currently running the latest version), changes/removes Thunderbird as my default mail program. The problem is that none of the TB options are selected in CCleaner, and Thunderbird is my ONLY email client. What can I do to correct it?