hello halfdeaded, you can launch firefox in safemode by pressing the shift-key while it's launching, however i'm not sure if this will work in solving the issue.

if the site is just preventing you from navigating away you might want to disable javascript temporarily: open a new tab & enter about:config into the addressbar (confirm the notice). in there search for the preference named javascript.enabled and double-click it in order to change its value to false. then it should immediately be possible to close the scam-site in question. afterwards turn back the javascript.enabled preference...

in case the site opened by its own, there already might be malware present on the computer. please go to firefox > addons > extensions & remove any suspicious entries (toolbars, things that you have not installed intentionally, don't know what purpose they serve, etc). also go to the windows control panel / programs and remove all toolbars or potentially unwanted software from there and run a full scan of your system with the security software that you have in place and different other tools like the free version of malwarebytes & adwcleaner.

Remove a toolbar that has taken over your Firefox search or home page Troubleshoot Firefox issues caused by malware

You can also "Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

You can delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox profile folder to prevent the previous session from restoring.

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Multiple_profile_files_created

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).