You can look at the browser.link.open_newwindow prefs on the about:config page.

• http://kb.mozillazine.org/browser.link.open_newwindow.restriction (0)
• http://kb.mozillazine.org/browser.link.open_newwindow (3)

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.

Can you give me the value to enter once I enter about:config?

Options window - Tabs panel

• https://support.mozilla.com/en-US/kb/Options%20window%20-%20Tabs%20panel

You can also middle-click or CTRL + left-click a link to open it in a New Tab

Check and tell if its working.

Filter: browser.link.open_newwindow.restriction
Value: 0