As a last ditch attempt you could try https://addons.mozilla.org/en-GB/firefox/addon/multi-touch-z...
But if that doesn't work it may hint that pinch gestures aren't being passed to firefox properly; try pinch zooming on maps.google.com, if that doesn't work then maybe there's a deeper issue going on
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
Bugs from a quick play-around (comparing with Microsoft Edge):
* The zoom with touch pad has a weird latency not present in Edge.
* Zoom on touchscreen is janky and jumps around.
* Zooming in and then out often (but not consistently) stops working half way through the gesture.
* Scrolling behaviour while zoomed is wildly different to native apps like Edge. Scrolling is extremely fast and has some kind of acceleration.
* Pinching while scrolling on the touchscreen lags badly.
It just feels shitty. Firefox devs don't care about a polished experience on Windows touchscreen laptops and it's a dealbreaker for me.
Maybe check your touchpad and touchscreen drivers.
I haven't been using Firefox for nearly 20 years because of a rendering engine, I've used it because of its features. In the last 20 years, most people have moved from desktops to laptops and phones, native app battery use and performance means a lot more than features.