Why does my Firefox open two windows when launched by URLs in other applications?

I'm using Firefox v3.6.6 as my default browser on OS X 10.6.4. For a while now I've noticed that whenever I open a URL from another application (e.g., PDFs in Preview, Steam, OSX help pages, Terminal) two Firefox windows are opened, with the requested URL in the second.

If Firefox is already open this doesn't happen; the URL is opened in a new tab in an existing window, as per my Firefox preferences. The symptom doesn't occur with Safari as the default browser. Similarly, no matter what the default browser, open -a Firefox http://superuser.com gives the double window whereas open -a Safari http://superuser.com does not.

For the life of me I can't work out why this happens. Is there an OSX plist or Firefox about:config setting that I should be checking?

From superuser

• It's not you; it's your browser.

  Deditos : That's a first; it's usually me. Thanks.