Well, now that update.mozilla.org is finally responding to requests in a timely fashion, I was able to find out about an update for the Single Window extension that I used to use, and just in time too, as having new Firefox windows (not tabs) popping up now and then was starting to annoy me.. only took a day.
It turns out that the functionality is now inbuilt into Firefox, heres a quick rundown on how to turn it on (taken from the Single Window extension info page):
Firefox 1.0 has built-in single window functionality. To enable this feature type "about:config" in address bar and set "browser.tabs.showSingleWindowModePrefs" to "true". Then in "Tools/Options/Advanced", enable "Force links that open new windows to open it" option. Thats it.