https://developer.mozilla.org/en-US/docs/Creating_MozSearch_plugins
https://developer.mozilla.org/en-US/Add-ons/Creating_OpenSearch_plugins_for_Firefox
https://nl.wikipedia.org/w/api.php?action=help&modules=opensearch
The folling file was used for creating the icons. It was enlarged to 1280x1280, cropped to 1150x1150 and rescaled to 64x64 PNG and 16x16 ICO.
The following file was used as screenshot for the add-on site.
https://nl.wikipedia.org/static/images/project-logos/nlwiki.png
The following file was used as screenshot for the add-on site.
https://upload.wikimedia.org/wikipedia/commons/d/df/WikiPedia.png
The following file was used as screenshot for the add-on site.
wikipedia-nl-homepage.png
The folling file was not used. (Was used for icon.)
https://nl.wikipedia.org/static/favicon/wikipedia.ico
The folling file was not used. (Was used for site icon.)
https://upload.wikimedia.org/wikipedia/meta/0/08/Wikipedia-logo-v2_1x.png