You can download latest version 2.0.121 from the WebArchive, but it’s not so straightforward, so follow these steps:
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right click “Download file”
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in the context menu select “Save Link As…”
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this will download “enhancer_for_youtube-2.0.121.xpi” file
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open “about:addons” page and click the gear icon in the corner and select “Install Add-on from File…”