Right click task context menu behaviour inconsistency with Firefox on a zoomed https://quire.io webpage

When https://quire.io webpage is zoomed in/out (not 100%) in Mozilla Firefox 57.0 (tested on Linux Mint 18.3 KDE edition) and when clicking on a task with right mouse button the context menu mostly shows for 1 second and disappears, in some rare cases it stays open as would be expected ... With Chromium, on the same system, it works fine ...