Click Modifiers is a free, open source extension which allows you to change the action that occurs when you click on a link while using a modifier key or middle mouse button.
Change the action that is performed when clicking on a link while using command, control, alt, shift, or meta modifier keys as well as middle mouse button clicks. Note though that on some platforms the actions for some keys can not be modified, for instance the action for a control click on MacOS X can not be changed. Shift key can also be used along with other modifier keys to toggle between opening links in a new foreground or background tab. Actions for clicks can be set to perform the default Firefox action, open links in the current window/tab, open links in a new window, open links in a new tab, open links in a new background tab, or save the link. Installs without needing to restart Firefox. Works on Firefox v10 or newer. Works on MacOS X, Windows, BSD, Linux, and Unix platforms. Completely free and open source.