Earlier quoted context omitted.
When you pinch does it jump between zoom levels in large increments or is it smooth? The large jumping was the old behavior – check about:config's browser.gesture.pinch settings are all empty and not mapped to ctrl+ or anything. I haven't tried on Windows but on macOS the native zoom is smooth and pretty good As a last ditch attempt you could try https://addons.mozilla.org/en-GB/firefox/addon/multi-touch-z... But if…
It no longer jumps in big increments but it's still janky and very much non-native.
If it's janky it might "just" be a performance issue. If you could grab a profile from profiler.firefox.com that would be much appreciated