Replace element `instanceof` checks with `.localName`

Replace element `instanceof` checks with `.localName`

KISS.

Note: `instanceof` checks are still often useful for XUL elements, because some
interfaces are extended by several elements.

Also note that `.localName` is used instead of `.nodeName` because many XUL
pages embed HTML elements. When mixing namespaces like that, the `.nodeName`s of
HTML elements will start with `html:`, while `.localName` skips the prefix.
`.localName` also nicely always returns valid HTML node names in lowercase.