Sorry to hear about this problem. Usually when you have dialogs that appear in a page and prevent you from leaving or using the interface, you can escape using one of these methods:

(A) Two-line dialog asking for username and password: press the Esc key numerous times to break the cycle

(B) Very tall dialog with text: look near the bottom left for a checkbox that says you do not want any more dialogs from the page, check that box, then click OK

These pages seems to be launched from "malvertisements" (ads driving users to malware distribution pages). If you haven't considered using an ad blocker, perhaps this is the time. This one is popular:

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

When you crash Firefox to shut it down, of course, Firefox will attempt a crash recovery. When there is a site you want to avoid, you may need to hide your session history files from Firefox so that it can't restore the unwanted page at the next startup. Here's how:

Windows

Type or paste the following into the Windows Run dialog or the system search box, and press Enter to launch Windows Explorer into this folder:

%APPDATA%\Mozilla\Firefox\Profiles

You should see at least one semi-randomly-named profile folder there. (If you see more than one, you'll want to figure out which one has the unwanted session history in it by checking each one.)

• Double-click into the profile folder

• Right-click the sessionstore-backups folder and rename it to OLDsessionstore-backups (later you can mine the files in this folder for any important tabs you want to restore)

• Check for any files with these names and:
  • sessionstore.jsonlz4 - created by Firefox 56 when it shuts down normally - rename to OLDsessionstore.jsonlz4
  • sessionstore.js - created by Firefox 55 and earlier when it shuts down normally - rename to OLDsessionstore.js
  • sessionstore.bak - obsolete file from years ago - delete

When you start Firefox up again, it should just show the home page.

Success?

If you need to read the contents of any of those hidden files, you can use this tool on my websites to extract out a list of the tabs:

https://www.jeffersonscher.com/res/scrounger.html

Mac or Linux

Your Firefox profile folder would be accessible in a different way than on Windows. Please see these articles for information on how to locate it:

• Profiles - Where Firefox stores your bookmarks, passwords and other user data
• http://kb.mozillazine.org/Profile_folder_-_Firefox

After that, continue with the second bullet point in the above process.