Most likely, it was just a scam message that was pushed out through an ad in a page you visited. Unfortunately Firefox's phishing and malware protection is based on lists of bad sites. Because these scammers keep creating new sites, it is difficult for the feature to keep up. There are a few common patterns to these annoying pages, and these are some techniques for closing them without having to take drastic measures. The "key" (ha ha) is the keyboard shortcut for closing the current tab, which is Ctrl+w (or on Mac, Cmd+w). Try it after each action to see whether it is available yet. (1) Large alert dialog (lots of text) If you cancel this dialog, it may reappear. After two or three appearances, Firefox should add a checkbox at the bottom of the dialog to stop the site from showing more alerts. Check that box and click OK to block further dialogs. (2) Authentication dialog (asks for username and password) If you cancel this dialog, the page may reload and immediately show it again. Pressing the Esc key numerous times in a row can cancel the reload as well as the dialog. (3) Reacting to mouse movement Some pages have a script that detects when you are moving the mouse pointer up toward the tab bar and takes action to show another dialog, or moves to full screen view to hide the toolbar area. On these pages, the keyboard shortcut is essential. Hopefully this will let you close problem pages without having to "kill" Firefox in the Windows Task Manager. (I don't recommend using that method because the tab usually will come back during automatic crash recovery anyway.) Since these scam pages often are promoted through ad networks, a defensive measure you could consider is using an add-on that is effective at blocking ads, such as: https://addons.mozilla.org/firefox/addon/ublock-origin/ As with any content blocker, this will cause problems on some sites, so keep an eye on its toolbar button in case you need to make an exception to get a page to load properly.

P.S. Good luck with the soup!