contextMenuShowEvent

A BrowserSetting object which determines whether the browser's context menu is shown on the mouseup event or on the mousedown event.

Its underlying value is a string that may be either "mouseup" or "mousedown".

The default value is "mouseup" on Windows, and "mousedown" on macOS and Linux. Assigning to it on Windows has no effect - the setting is only designed to allow the context menu to be opened on mouseup instead of mousedown, not the inverse.

Set the setting to "mouseup":

function logResult(result) {
  console.log(`Setting was modified: ${result}`);
}

browser.browserSettings.contextMenuShowEvent.set({value: "mouseup"}).
  then(logResult);