Firefox has two forms of zooming, one of which resemble what you're describing.

1. Zooming with Ctrl++ or Ctrl+- will "tell the page" that it's being zoomed and it will reflow into the new viewport size.

2. Zooming with two-finger pinch (requires a touchpad) will zoom in as if magnifying the page. Content on the page will overflow to the right instead of reflowing.

Supporting an equivalent on regular desktop windows could be pretty useful.