There’s definitely ways to know if they really wanted to stop that, but those employees aren’t going to pull something like that. They weren’t just told they can’t use Mozilla, they were told they must use Edge. Using anything else is noncompliance (which I absolutely support as a person but as an employee I have no say in the policy)

Besides, with the upcoming changes to 365, you’ll never get links to open in anything but Edge without admin credentials at the very least, but realistically even that won’t stop it. You could use a portable version I suppose, if just to have at least one browser with proper uBlock support.