Browsers are on a war path. First, in case you were wondering why editing emails in workflows no longer work, that’s because Chrome disabled showModalDialog, breaking bucketloads of CRM functionality relying on that. See this good summary for a comprehensive feature list and a temporary workaround.
Firefox has been much sneakier than that. It kind of works but every now and then drops the checkbox on unsuspecting user, that is way too easy to accidentally click, press OK and render your CRM useless until the browser restart.
To stop Firefox from ever sneaking this option in: