I checked settings were correct and that drivers were up-to-date to no avail. A Google search for "firefox right click menu flickers" produced the following suggestion:-
Try to disable hardware acceleration.Untick box and restart Firefox, this solved the problem.
If disabling hardware acceleration works then check if there is an update available for your graphics display driver.
- Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" .