Someone hijacked your "Download MP4 360" link and is blocking browsers, attempting to upload code - some call it "malvertising" under the pretext of a browser (Firefox) "Important update". Firefox is 74.0 (last update, and up to date). Thank you. C. S.

This is repeatable:  When clicking on your "Download MP4 360" (see example further down) this "page" comes up, as explained in the Mozilla support page at:

https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update  

Not sure if it happens also with all other browsers, seems they have a variant for Chrome, but I could close the browser.  With Mozilla, I can't do that, it "vibrates" the pop-up and I can't do anything, they want me to click and download their garbage.  The way I got out was to use the "Process KO" an independent tool, to kill Firefox.  Else the PC needs to be forced-shutoff. Here's an example of page that leads to this lock-up:

https://www.ssyoutube.com/watch?v=dpB8GfnbQ3o

then click on "Download MP4 360".  For me, it leads to a page by "katydogs.com" that locks my Firefox browser.  Whatever I do, including attempt to shut off Firefox, results only in the flashing/vibrating of their popup.

I hope this helps,

CS

Screen crop below:

Check your computer for viruses.