Notification saying "A new Virus is Stealing Data!" on Ubuntu 18

Recently whenever I open Firefox I get a message in notification center. It's not always the same and it keeps changing. here is an example:

It's not always this. It gives other messages too and when I click it it sends me to phishing sites. I don't know how I got it.

Could someone please help me get rid of it?
I tried Clam antivirus but it found nothing.

Solution 1:

Keep calm, it looks like a mere phishing attempt with FUD. It seems some website(s) somehow tricked the user into giving permission for push notifications.

To fix this, follow the steps below.

1. Open Firefox, click the "hamburger" menu button (three horizontal bars), and choose Preferences.
2. Select the Privacy & Security panel and go down to the Permissions section.
3. Click the "Settings..." button next to Notification and look for dodgy-looking websites which are "allowed".
4. Select the website(s) and click "Remove Website".