Comment 17 for bug 272171

Fount it. There is a setting browser.tabs.loadDivertedInBackground in about:config , just set this to true, and firefox won't steal your focus any more when loading links in background.

(Thanks to this site http://www.techsupportalert.com/content/how-stop-firefox-stealing-window-focus.htm for pointing this out)

However, it won't highlight the window in the window list either, making it hard to locate the window if you have many desktops.

But maybe, this is enough to point some developer into the right direction how to fix it properly (or how to explain to us what is going on behind the scenes, so that it can be fixed properly in the window manager, if it is the window manager who screws this up)