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Open the Windows search bar. Type notepad into the search bar. Right-click Notepad in the search results. Click Run as administrator. Click the File menu. Click Open. Type or paste the full path to the hosts file into the “File name” field. Click Open. Place your cursor at the bottom of the page. Type 127.0.0.1 at the bottom of file. Press Tab ↹. Type the address of the site you want to block. Click the File menu and select Save. Restart your PC.
If you don’t already see the bar to the right of the Start menu, click the circle or magnifying glass icon that appears there to open it. This method will block a specific website from all browsers, not just Firefox. Don’t press anything else. A list of search results will appear. A menu will appear. This opens Notepad as an administrator, which allows you to edit the file you’ll use to block a website. If prompted to confirm, click Yes . It’s at the top-left corner of Notepad.  Typically the path is C:\Windows\System32\Drivers\etc\hosts. If you’ve installed Windows on a different letter hard drive (e.g., D), replace the drive letter in the path. This opens the hosts file in editing mode. Scroll down to the bottom of the file, then click below the bottom item. You’ll be adding text to its own line at the bottom of the file.  This inserts a space. For example, if you want to block YouTube, type www.youtube.com. If you are prompted to confirm the save, follow any on-screen instructions to do so. Once your PC comes back up, the website will be blocked.