It sounds like you got a random page claiming to have a Firefox update. This is a scam.

Firefox updates are done internally or by download from mozilla.org like www.mozilla.org/firefox/all/

There was new update Released yesterday 57.0.3

Unfortunately, ads sometimes drive redirects to sites distributing dangerous malware.

Pages designed to trap you typically use a couple different methods to make it difficult to close them. I suggest trying:

(1) Tap the Esc key numerous times in a row to close any dialog asking for a username and password, to break the reload cycle.

If you are asked whether you want to Resend or Cancel some data you submitted on such a site, Cancel (or Esc).

(2) For large script dialogs, look for a checkbox at the lower left to stop the page from showing more dialogs. Check the box and click the OK button. Then quickly press the Esc key numerous times.

(3) For pages that watch you moving the mouse around and then show something else, try using Ctrl+w (Close tab) or Ctrl+F4 (Close document) to close the tab.

And...

Since these often pop up from bad advertisements, consider using an ad blocking extension like uBlock Origin: https://addons.mozilla.org/firefox/addon/ublock-origin/

Some websites have incomplete functionality when ads are blocked, so when a site is trustworthy, you can use the uBlock Origin button on the toolbar to make an exception for that site.