. Whenever you get a message / popup like that;

DO NOT USE ANY OF THE PROVIDED LINKS OR ANY PHONE NUMBERS ! !

Such messages are only Spam that could lead you to Fake Support. Not only can they damage or hack your system, they charge you to do so.

You can report such a site at; Google Report Phishing Page {web link} which is the same when done while on site by going to Help > Report Web Forgery

Help us safeguard Mozilla’s trademarks by reporting misuse {web link}

Mac Malware Scanners https://discussions.apple.com/message/29938930#29938930

You can also try https://www.malwarebytes.org/antimalware/mac/index.html

I suspect that if you force quit Firefox, the session restore that normally appears after a crash will bring that unwanted page back. To break the cycle, you could try one of these methods:

(1) Remove the session history file so Firefox can't restore that tab (2) Change a setting so that Firefox shows a list of tabs and you can deselect that one and restore the others

Both of these require a little manual work with Finder, so let's start there. You need to open your current profile folder, which unfortunately is a hidden folder. Please see this article for the steps for Mac:

Profiles - Where Firefox stores your bookmarks, passwords and other user data

For #1:

(A) In the profile folder, you may or may not find these files:

sessionstore.js sessionstore.bak

If you find those, either move them away from the profile folder, rename them so Firefox won't recognize them, or delete them. As an example of how to rename, you can edit the names to sessionstore-old.js and sessionstore-old.bak.

(B) Rename the sessionstore-backups folder to something like sessionstore-old so you can refer back to these later if you like.

Having hidden all of Firefox's session history files, you now should be able to start up to your home page.

For #2:

In the profile folder, find the prefs.js file and open it in a text editor. It should consist of numerous lines similar to the following, which is the line you want to add to the end of the file:

user_pref("browser.sessionstore.max_resumed_crashes", 0);

Save and close the file. When you start Firefox, it should present the "this is embarrassing" screen allowing you to select which tabs to restore and not restore so you can avoid the bad one.

(If you do both #1 and #2, then you'll only see the result described under #1)

If it's like the fake FBI/Interpol browser lock page, This add-on can stop such pages; disallow Script Button {web link} The Disallow Script button looks like a letter "M" and the title is the Minus Script, drag and drop the button on a toolbar. If the button is not displayed then nothing operates, except rules for plugins.

Another way out of the trap is to close Firefox (force it if needed). Then right-click the Firefox icon and select Open New Window.