Improve Firefox support

[japsu]

Press FF to doubt.

• Lazy loading: Firefox does not support loading="lazy", so make up some JS fallback.
• Preloading: Our current approach of floating <img> tags created via JavaScript does not trigger pre-loading in FF.
• Alt texts visible in blue while loading thumbnails: Perhaps just make alt texts (link text) grey or sth.
• Make sure WebP is used on FF 65+: Last I checked, image quality was potato, which can be expected on JPEG but FF should support WebP nowadays. Make sure our WebP detector correctly detects Firefox as WebP capable.

[japsu]

Revisited: New method of WebP detection works reliably on FF but the other three subtasks still apply.