On the forum/node listing page, all the categories have got section anchors (href="#category-title"). Expected behavior on clicking those links is to scroll the page enough to bring that section at the top of visible region. It seems like for non-ASCII characters, it is using some character replacement/key map technique to dynamically create its ASCII equivalent. And failure to find a key map, leads to URL encoded character equivalent. In such cases where not all characters have got key map equivalents, expected behavior is not seen on Firefox. Although, it works on Google Chrome.