A: Sizzy only simulates viewport size, pixel ratio, user agent, and UI—it does not simulate different browser engines.

For example, setting the user agent to Firefox while Creating Custom Devices or Cloning Devices still renders the page using the Chromium engine instead of Firefox's Gecko.

It's still way faster than firing up many simulators at once, and it gets you 90% there, but there might be minor differences between how the website looks in Sizzy vs how it looks in a "real" Firefox browser.