After some testing, I tried to generate a download of a savegame contained in the IndexedDB storage but the file was not properly formatted, something went wrong with file size, though lcf headers were valid. Since Firefox 49 this data can be deleted from the storage inspector, but there is no easy way even for developers to dump this data.

A documented and (hopefully integrated) working way to export these savegames may be useful for us as even for end users to import/export savegames, for testing purposes and for fun.