You *think* - or you know?
you don't need anything extra. In Thunderbird you mark / select the files you want to copy. Then go to "Files" and click Save As... > Files, and navigate to the folder > Save. That's all.
Well, it certainly can be done as an add-on. Evernote used have one (maybe still do). I currently use an InfoQube addon to save emails to an InfoQube folder (but it's not customisable, sorry!)
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