I’m not sure why one would want this feature. But the fact that this is achievable and one of my visitors asked for it tells that there are people who are looking for this.
Disabling Tab Feature in Firefox is very easy. Install Tab-Killer extension from Mozilla Add-on database and you are set to go.
The Tab-killer extension ensures there is only one tab in each instance of Firefox running on your system. If you click on a link, the link will open in a new window.