From MozillaZine Knowledge Base

The option for opening links in new tabs with a middle-click (or Ctrl+click) is off by default. To turn it on:

1. Go to Edit -> Preferences
2. Expand Navigator (or Browser in SeaMonkey 2) in the left pane.
3. Select Tabbed Browsing
4. Check "Middle-click, Control+click or Control+Enter on links in a Web page".