But why? There's no need for a PDF editor in a browser. This is just raising the size and lowering the performance of a browser for a feature that almost no one needs or wants.
Mozilla should probably build a PDF viewer/editor (Adobe sucks and Foxit has gone downhill in recent years), but it's not part of Firefox.
It has nothing to do with website design. It's part of the HTTP protocol. A poor part in today's understanding and use cases, but in the 90s it would have made sense.