Many websites contain links that cause your Web browser to open new windows when you click them. Mozilla's Firefox browser has settings you can configure to force websites to open in new tabs within the same window instead. Having multiple open windows uses more of your computer's available memory than a single window with multiple tabs.
Open the "Tools" menu at the top of your window. Select "Options..."
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Click the "Tabs" icon at the top of the "Options" window.
Click the check box next to "Open new windows in a new tab instead." Click the "OK" button at the bottom of the "Options" window to apply your new settings.
Though having multiple open tabs in a single window uses less memory than multiple open windows, if enough tabs are open simultaneously, you might experience slower response times from the operating system. Click the "Warn me when opening multiple tabs might slow down Firefox" check box in the "Options" window to get advance notice.