If a website codes its links to open in a new window, you can divert that to a new tab. The basic setting for this is here: "3-bar" menu button (or Tools menu) > Options > Tabs The first checkbox: "Open new windows in a new tab instead" In some cases, a website uses a script to launch a new window with special features, such as a set height/width, or with one or more toolbars removed. To avoid breaking sites that create dialog boxes this way, Firefox does not normally divert this kind of window to a new tab. However, if you wanted to send these to a tab as well, there is a hidden setting you can change. (See: https://support.mozilla.org/questions/1035103#answer-662627]) If Firefox is sending ALL links to new windows or new tabs, you might have an extension like Tab Mix Plus or Tab Utilities which changes the built-in tab handling features. You would need to explore their settings to get it just the way you want it.