Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller (for more info, see this alternate TDSSKiller download page)
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista if you don't already have one. Windows 8/10 have antivirus protection built-in.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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First, it's probably not true that your computer is infected. You should check, but let's try to get Firefox "unblocked."

There are a few common patterns to these annoying pages.

(1) Large alert dialog (lots of text)

If you cancel this dialog, it may reappear. After two or three appearances, Firefox should add a checkbox at the bottom of the dialog to stop the site from showing more alerts. Check that box and click OK to block further dialogs.

(2) Authentication dialog (asks for username and password)

If you cancel this dialog, the page may reload and immediately show it again. Pressing the Esc key numerous times in a row can cancel the reload as well as the dialog.

(3) Reacting to mouse movement

Some pages have a script that detects when you are moving the mouse pointer up toward the tab bar and takes action to show another dialog to moves to full screen view to hide the toolbar area. On these pages, try using the keyboard shortcut Ctrl+w to close the tab instead of the mouse. (On Mac, Command+w)

Hopefully this will let you close problem pages without having to "kill" Firefox in the Windows Task Manager. (I don't recommend using that method because the tab will come back during automatic crash recovery anyway.)

Often these tech support scams are promoted through ad networks. As a defensive measure, you could consider using an add-on that is effective at blocking ads, such as:

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

As with any content blocker, this will cause problems on some sites, so keep an eye on its toolbar button in case you need to make an exception to get a page to load properly.

A way out of a trap page is to (force it if needed) Close Firefox. Wait about 20 seconds for Firefox to finish closing. Right-click on the Firefox icon and select Open New Window. +++++++++++++++++++++++++++++++++ Sometimes this works. If it opens, close it by Menu > File > Exit. This will flush out that web page from the Session Store.