Open a new tab/window in an existing Firefox instance on Linux

I would like to open a URL from the command line and have this open as a new tab or window in an already running and responding instance of Firefox. I'm using Debian's Sid branch. If I do the following:

In term 1:

$ firefox

In term 2:

$ firefox 'http://www.google.com'

Update: I've also tried -new-tab and -new-window, but I get the same result.

$ firefox -new-tab 'http://www.google.com'

I get the message:

Firefox is already running, but is not responding. To open a new window, 
you must first close the existing Firefox process, or restart your system.

This has been discussed before (e.g. here and here), however, I do not want to kill the existing instance of Firefox (it's doing just fine) and I don't want to use a different profile. This works seamlessly with Google Chrome.