It’sa feature of safari, long press the Aa button on your address bar and most (decent) websites will show reader versions (technically any well formed html page should do it, but there are dastardly tricks to prevent it and pump more ads at users).

It’s also shown in external pages using voyager’s web app, but you can’t make it default like Apollo could.

In firefox its on the right hand side in the adress bar, left of the zoom. A little page symbol.