Rated 4 out of 5 stars

I guess the dev should look into this to address the recent error in drag and drop

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Drag and Drop API

The DataTransfer.items property has been implemented, allowing access to multiple items being dragged and dropped using the HTML Drag and Drop API. To allow this, the DataTransferItem and DataTransferItemList interfaces are now supported as well (bug 906420). This is enabled by default.
The old, obsolete Firefox specific drag and drop API events dragdrop and draggesture are no longer supported. Be sure to update any code still using them to use the HTML drag and drop API (bug 1162050.
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This review is for a previous version of the add-on (2.0.29). 

This will be fixed in version 2.0.30 - sorry for the inconvenience. You can downgrade to the previous version in the meantime.