We've faced an issue with menu on several RAD controls when running on MacOS 10.14.x (Mojave) with FireFox 66.0.x (Quantum).
Using touchpad or mouse, when right clicking for context menu, it just flashed and doesn't stay open: it opens and then suddenly closes.
We are using Telerik Asp.Net Ajax version 2017.2.503.40 but it can be also replicated on your demo version:https://demos.telerik.com/aspnet-ajax/grid/examples/columns-rows/columns/context-menu/defaultcs.aspxhttps://demos.telerik.com/aspnet-ajax/scheduler/examples/overview/defaultcs.aspx
Not sure if you can provide a fast workaround for this but I just wanted to let you know.
I've checked for some open cases before creating this thread and hope I didn't duplicate something. Please move/close it if necessary.