hi artydeb, this is no genuine error message but a fraud trying to get you to install malware or spend money.

try the following to close the page:

• press f12 in order to open the dev tools
• click on the cogwheel icon on the top right of the dev tools
• check the option to disable javascript in the advanced settings section
• afterwards it should be possible to leave the offending site & you can close the dev tools panel again

If Philipp's solution does not fully fix this then a couple of other things to check & try

1. Has it reset your home page ? or is just opening the last page you were on.
   • How to set the home page
   • Startup, home page, tabs, and download settings
2. Has it managed to change shortcuts used by icons ?

If it is the shortcuts that have been changed, I think right clicking them in Windows gives options to see and edit them. Has it added something after Firefox that should not be there. Temporarily that may be circumvented by opening Firefox from a run command. Keyboard shortcut Winkey+R shold get a run dialogue. Typing in firefox.exe and pressing Enter should start Firefox normally.