Thanks Leo and Jeremy for the reply. Disabling other firefox plugins and using empty.zip from tiddlywiki.com, the problem remains. Now I found this issue only happens when the tiddlywiki file is saved on my virtual drive (google drive + Boxcryptor). If I copied those files to my local disk. The problem resolved. I think it might be something to do with Boxcryptor. I'll change different Boxcryptor versions and see if that makes a difference.
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