cancel-duplicate-downloads от KerfuffleV2

Simple addon to cancel auto-renamed downloads that look like "filename(1).ext"

Click the horribly ugly extension icon to toggle enabled status.

The addon will display a badge with the number of cancelled downloads. SHIFT-click the icon to reset the count. The badge will be red if the last download was cancelled.

Note:
1. Does not actually check if the file exists.
2. Does not apply to filenames without an extension.