Disable Insert Next to Current Tab in Firefox

1. In Firefox, type about:config into Location bar
2. Click on I’ll be careful, I promise! button
3. In the filter text box, enter the following text string: browser.tabs.insertRelatedAfterCurrent
4. The default boolean value for the browser.tabs.insertRelatedAfterCurrent is “true”. Double click on the browser.tabs.insertRelatedAfterCurrent to change its value to false
5. Close the tab, and the change will take effect immediately