Steps to reproduce: Double-tap-drag zoom in and out (and repeat a few times). Expected result Smooth zoom in and out, and page ends up in the same position. Actual result When the zooming works, there is a lot of checkerboarding going on (white flashing areas), and sometimes the page's coordinates seem to jump around / be reset to 0,0. In some instances, the page display is completely broken, and a refresh is required to continue browsing. Environment: Firefox 55 beta on Nexus 6P
Can you provide a specific URL on which you see this? The coordinates "snapping" is a preexisting issue that happens with regular pinch zooming as well, it is tracked by bug 1358771 (assuming that is what you are seeing, it's hard to tell from your description). A video would be quite valuable here.
Sure, I see it here: http://andreasbovens.github.io/understanding-viewport/noviewport.html
I've landed a few fixes (including bug 1358771) to the one-touch-pinch behaviour since this bug was filed. Are you still seeing the janky/jumpy behaviour?
I see less jumpiness, but triggering one-touch-pinch zoom is still inconsistent. It typically works 1 time out of 5 or so.
That's bug 1387055, so I'll close this one.