I finally got fed up with Firefox (I'm using Firefox 11.0) opening external links (that is, links launched from other programs, such as by clicking a link in an email in Outlook) in a new tab in the current window instead of in a new window and so I figured out the setting to change to make external links instead open in a new window: browser.link.open_newwindow.override.external. I can't find this documented well anywhere and thus I'm writing this blog post in the hope it helps future searchers for the solution to this same issue.
In my install of Firefox, the value of browser.link.open_newwindow.override.external was by default -1. After some trial and error, I found that by setting it to 2, external links open in a new tab.
And to be clear, this is in the situation where I have Firefox set so that new pages opened from within Firefox open in new tabs (except in the case of popup windows: there's another setting for those). If you simply want all new pages to always open in new windows instead of new tabs, you don't need to set browser.link.open_newwindow.override.external: all you have to do is uncheck "Open new windows in a new tab" in the Tabs tab of Preferences and you'll be set to have ALL new pages always open in new windows, no matter if they're opened from within Firefox or from external sources (again, with the exception of popups for which there is another setting, specifically browser.link.open_newwindow.restriction).
If you don't know how to access these settings I'm talking about, the short story is that you type about:config in your Firefox's address bar. The longer story can be found at http://kb.mozillazine.org/About:config.
By the way, I make use of multiple tabs in Firefox, I just don't use them for random collections of pages: I use multiple tabs to open multiple pages related in a way I deem important. For multiple pages that are not related in a way I deem important, I use different windows. This keeps things organized effectively in Firefox for me. So since, more often than not, the new pages being opened in new tabs are not related to the other tabs in said windows, it makes more sense to open them in new windows than it does to open them in a new tab in an existing window.