From the bugzilla[0] about Pinch to Zoom last year: "We definitely haven't forgotten about this, and we are working on it (as other priorities permit)."

[0]https://bugzilla.mozilla.org/show_bug.cgi?id=688990

If you are on 69 or higher, go to "about:config" and enable "apz..allow_zooming". It's still a little buggy, but it works quite well