The cache in Thunderbird does the same as in Firefox - it caches web content. In any case, it is strongly recommended not to let CCleaner mess with Thunderbird files. It can cause other undesired effects otherwise. Uncheck all Thunderbird options in CCleaner. If you want to clean the Thunderbird cache do that from within Thunderbird.

Well, actually, the reply does somewhat help? The reply reinforces not using CCleaner for Thunderbird. But I'm still not clear on what 'web' content could be associated with my email files? I understand cache when associated with Firefox, to allow supposed faster site loading. But emails?? When clearing cache files from within Thunderbird, what is impacted? If anything? If nothing, then why cache?

I'm still not clear on what 'web' content could be associated with my email files?

I suppose you've heard about HTML email? A message may contain images dynamically loaded from a server, as opposed to attached to the message. Another example are RSS feed items, which are basically web pages.

I understand cache when associated with Firefox

Wrt cache Thunderbird is no different than Firefox.

When clearing cache files from within Thunderbird, what is impacted?

Well, you clear cached items, which will then need to be reloaded if you access them once again. It really is the same thing as with Firefox. In case it helps to understand, Thunderbird basically has the same web browsing capabilities built in as Firefox.

Obviously I never associated it that way. My options are set to send in HTML format, but not associating any web content within any of my outgoing, or remembering any HTML web based content in any incoming, I'll just do the clean up without any concerns. Thanks for your clarifications.