Prevent windows from stealing focus

The setting is in CompizConfig > General > Focus & Raise Behavior as so documented. The Window Matching rules are quite important.

This is not something you should configure globally. I.e. you don't really want no new windows to take focus: you want no test windows to take focus. To accomplish this, use the Compiz Window Rules plugin to prevent all "test" Firefox windows from taking focus.

You would need to write a rule to identify only "test" Firefox instances. If they all have a specific string in the title, that would work. Otherwise you could use wmctrl to set the WM_ICON_NAME X property. (I don't know how to easily set X properties for Firefox .. if anyone else does, please chime in!)