Firefox - "Open in new Tab"

[_m3]

Disable target="_blank"
Firefox has a built-in pop-up blocker that is activated by default. However, it doesn't stop web pages from opening in new windows by using the target="_blank" or the illegal target="_new" properties. To force such links to open in the same window, add the following code to your user.js file:
// disable target="_blank" (open in same window):
user_pref("browser.block.target_new_window", true);
http://texturizer.net/firefox/tips.html#beh_blank

Tabbrowser Preferences extension for Mozilla Firebird
This extension enables "single window mode" by optionally opening links in a new tab instead of a new window. It also gives an option to load the homepage in new tabs.
http://www.intraplanar.net/projects/tabprefs/

Tabbrowser-Extensions
The Single Window Mode, which prevents to open other browser windows, is available. In this mode, any windows are opened in the window as tabs.
http://white.sakura.ne.jp/~piro/xul/...s.html.en#what