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You’ll find it in the All Apps area of the Start menu in Windows, and in the Applications folder in macOS.  If you’re experiencing slowness, it may be the result of a cached item, bad cookie, or a large web history. Use this method to clear these options. Clearing cookies will log you out from any websites you have open. It’s at the top-right corner. It’s near the middle of the menu. It’s in the left panel. It’s under the ″Cookies and Site Data″ header in the right panel. Check the boxes next to ″Cookies and Site Data″ and ″Cached Web Content″ to select them both. The amount of space occupied by each type of data appears next to its name. A confirmation message will appear. The cache and cookies are now clear. It’s under the ″History″ header. Choose Everything from the drop-down menu at the top of the screen, and then check all boxes. This ensures that your entire history is cleared, not just the sites you’ve visited most recently. Your history is now clear.

Summary:
Open Firefox on your PC or Mac. Click the ≡ menu. Click Options. Click Privacy & Security. Scroll down and click Clear Data. Select the information you want to clear. Click Clear. Click Clear Now to confirm. Scroll down and click Clear History. Select the information you want to clear. Click Clear Now.