You can hold down the Shift key and left-click a link or bookmark to open this link in a new window.

Also see: Tab preferences and settings

If you mean a shortcut or link completely outside of Firefox, there is a hidden preference to change how that works. This requires a visit to the about:config preferences editor. I think it's (C) below that you might want to change.

In a new tab, type or paste about:config and press Enter. Click the button promising to be careful.

In the filter box, type or paste link.o and pause while the list is filtered.

Double click these and set the value as desired:

(A) browser.link.open_newwindow

3 = divert new window to a new tab (default) (checked*)
2 = allow link to open a new window (unchecked*)
1 = force new window into same tab

* First checkbox in Options > Tabs

(B) browser.link.open_newwindow.restriction - for links in Firefox tabs

0 = apply the setting under (A) to ALL new windows (even script windows)
2 = apply the setting under (A) to normal windows, but NOT to script windows with features (default)
1 = override the setting under (A) and always use new windows

(C) browser.link.open_newwindow.override.external - for links in other programs

-1 = apply the setting under (A) to external links (default)
3 = open external links in a new tab in the last active window
2 = open external links in a new window
1 = open external links in the last active tab replacing the current page

Does that work?

This only seems to work from within firefox. I want to be able to run firefox from the command line specifying a URL, and if firefox is already running, have it open in a new window instead of a new tab.

Try the -new-window command line switch.

• https://developer.mozilla.org/en/Command_Line_Options

This was exactly what I wanted, to be able to specify on a case by case basis what happens. Thanks.