No, it's a scam, do not call them.

Pages like that, designed to scare and 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.

(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.

That phone number led to an interesting blog post from Microsoft saying you should not call it (or hundreds of others!):

https://blogs.technet.microsoft.com/mmpc/2017/04/03/tech-support-scams-persist-with-increasingly-crafty-techniques/

wondergirl said

I received this message in red that attempts were made to hack my accounts and that I should freeze my accts and call this number 855-xxx-xxx. I don't know if this is a scam or legit. How do I find out.

https://support.mozilla.org/en-US/kb/avoid-and-report-mozilla-tech-support-scams

Just a recent scam that some users have been encountering to try and trick less experienced Windows users into thinking they were infected and to call and give personal details and or pay for support.

Mozilla does not have the resources to do one on one support by phone, chat, or email for Free nor do they endorse any so called third-party tech support companies.