I'm running Firefox 51.0 (64-bit) on Debian GNU/Linux 8.7 (jessie).
The right click menu does not display when I right-click the page. This was happening before, stopped after I did an apt-get upgrade, and has no started again.
Instead of the normal right-click behavior, it alternates between two cursors, generally the text cursor and the pointer cursor.
This is reproducible on any site, including example.com
It happens even in safe mode, with a completely new profile.
There is nothing wrong with the hardware. Right-click works fine in other programs.
Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1344620