Firefox doesn't recognise the search engine; I don't know if they don't present it correctly, or if Firefox would need to approve the engine first, but it's not getting picked up.

You can add a keyword for search engines that are not recognised: right-click the search engine's search box/field and select Add a keyword for this search. Then use the keyword in your address bar to use that search engine and you should be set!

Firefox append a green + to the search engine icon in case a website makes a search engine available. If that isn't possible then you can use a keyword instead and right-click the search text field and click "Add a Keyword for this Search"