Firefox can be enhanced with a number of add-ons. But too many of them can slow down your PC. Also, too keep things exactly as you like them, other people who share your PC should not be allowed to install any add-ons on their own.

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You can completely prevent the installation of XPI files by configuring the advanced options. To do this, in the address bar, type 'about:config', press [Enter] and confirm the security warning. To search quickly through the long list, enter 'xpinstall' in the 'Filter' field. Now right click or double click the 'xpinstall.enabled' entry and select the context command 'Toggle' so that the value is changed to 'false'. The change is effective immediately. When an installation is attempted, Firefox simply stops it without any message.