http://www.kawaius.com/main_links/digital/ES100/ES100_brochure_EN_200dpi.pdf Try to read the 4th page "technical specifications" with a browser window about 900px wide (half a 1080p monitor). If you scroll down and then try to scroll to the side, your view gets reset to the top again. If you scroll right, then try to scroll down, your view resets to the left. You can't actually read the file if your monitor is not big enough. No idea if this a pdf.js or Firefox issue.
Note that scrolling to the right with the scrollbar, then scrolling down with a mousewheel does work correctly.
We would probably need exact STR for that, e.g. scroll with mouse or keyboard, settings of the zoom, etc. FWIW, I cannot reproduce on Mac OSX.
Just use the scrollbars and do exactly what Comment 0 says, everything else at the defaults (which seems to be Automatic Zoom).
Dup Bug 1059242 ?
I agree that sounds like a likely root cause.