Problem solved. At the suggestion of someone in FineTunedMac forums, I deleted the app from /Applications and also deleted the extension from ~/Library/Safari/Extensions. Then, I used
EasyFind to locate and delete stragglers. I then reinstalled the app and everything appears to be fine.

EDIT: After installing SZ, I found that Safari was very slow (lots of spinning beachballs). I unchecked Aggressive Mode in the SZ extension and that solved it.