You can set the browser.link.open_newwindow to 1 on the about:config page

• http://kb.mozillazine.org/about:config

See:

• http://kb.mozillazine.org/browser.link.open_newwindow (default: 3)
• 1: current tab; 2:new window; 3:new tab;

This pref set the behavior for JavaScript links:

• http://kb.mozillazine.org/browser.link.open_newwindow.restriction (default: 2)
• 0:all; 1:none; 2:only special windows

Hello iSkyiera

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise!, to continue to the about:config page.
3. At the Search area type(or copy/paste the next) browser.link.open_newwindow.restriction then, double-click on it and change the value to 0 (zero)
4. click OK.

Do the same with the : browser.link.open_newwindow and change the value to 1 (one)

see for more info :

http://kb.mozillazine.org/Browser.link.open_newwindow.restriction

http://kb.mozillazine.org/Browser.link.open_newwindow

thank you