This doesn't seem to work on the French version (google.fr) when accessed via a !gfr DuckDuckGo bang.

The extension metadata says this is published under the MIT license. Where is the source code hosted?

After the extension is disabled, its "Google Web" representation does not disappear from the menu of search providers.

Great addon but unfortunately the ability to use the All button link still doesn't work, despite claims this was fixed. Hopefully this can be rectified in a future update so I can change my rating to 5 stars. :)