Firefox Quick Tip: Reveal More Tab/Window Options
There are some hidden options for the Tabbed Browsing that will allow you to force links that open new windows to open in the current or a new tab. First, add the following code to your user.js file:
// Reveal more tab/window options:
Restart Firefox and go to Tools > Options… (Edit > Preferences… under Linux and MacOS X), select Advanced and click on Tabbed Browsing.