Because you do have something in there. Need to scan with anti-malware programs.

Please use more than 1 scanner as each uses diff tech :

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Save your Report and google each before deleting anything as do not want to delete something you need, If need help let the pros help, been there done that. :

• https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/

Post in only 1 forum, then wait.

Please let us know if this solved your issue or if need further assistance.

Often scammers will divert you to their "trap" pages from advertisements on popular websites. If you don't currently use a good ad blocker like uBlock Origin, you could consider it as a form of self-defense. As with any content blocker, some sites will not load completely unless you use the uBO toolbar button to make an exception.

https://addons.mozilla.org/firefox/addon/ublock-origin/

There are two commonly used styles for scam pages to trap you there, both of which include pop-up dialogs that are not considered pop-up windows and therefore not blocked by the pop-up blocker.

(1) Style #1: Repeated authentication dialog

There is a frame embedded in the page that demands a username and password. Each time you cancel it the frame reloads and displays the dialog again.

Press the Esc key several times quickly to cancel both the dialog and the reload. After that, you should be able to close the tab normally.

By close the tab normally I mean using Ctrl+w or Ctrl+F4 if you are prevented from using the mouse.

(2) Style #2: Repeated alert box

After you cancel the large panel, it comes back again and again.

Check along the bottom of the panel for a checkbox that instructs Firefox not to allow any more dialogs for the site, then click OK. After that, you should be able to close the tab normally. Example screenshot: