Browser Exit Warning

Browser Exit warning used to work well but now it seems to fail. It triggers an object but does not stop the page being left. I am using Firefox.
UPDATE
I've just done some testing amd browserExitWarning still works with Internet Explorer but not Firefox, Chrome and Edge.
UPDATE 2
Oops. Sorry. False alarm. I just read this on Stack Overflow "In Firefox you MUST do some real interaction with the page in order for this message to appear to the user." And yes, it's true so no further action required.
UPDATE
I've just done some testing amd browserExitWarning still works with Internet Explorer but not Firefox, Chrome and Edge.
UPDATE 2
Oops. Sorry. False alarm. I just read this on Stack Overflow "In Firefox you MUST do some real interaction with the page in order for this message to appear to the user." And yes, it's true so no further action required.