There are situations where you don't want a new tab to take focus in Firefox. Its been pain since you don't find any option under Firefox preferences to change this behavior.
Now there is a fix and you can force new tabs to open in background. Start your firefox browser, enter about:config in address bar and search for following key.
Now change this key to true by double clicking on it. If the above entry doesn't exist then create a new boolean key with above name and set it to true.
Enjoy Firefox. :)