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Mozilla Suite : FAQs : Middle click

From MozillaZine Knowledge Base

Middle click (or Ctrl+click) on a link does not open a new tab?

The option for opening links in new tabs with middle-click (or Ctrl+click) is off by default. To turn it on, open the Tools menu and choose Options. Navigate to Navigator | Tabbed Browsing catagory in the left pane. Then check Middle-click, Control+click or Control+Enter on links in a Web page.

Official 1.5 FAQ