Sometimes you don't want i3 to open a window in tiling mode. This is generally the case for popups or preference windows, like the one in Firefox (version 20). While most of them are correctly handled by i3, some programs don't set the right window type on those windows.
A way to fix it is to add a rule like this in your
for_window [class="Firefox" instance="Browser"] floating enable
To find out the class and instance of your window, launch xprop and click on it.
This will give you the window properties.
Look at the
WM_CLASS(STRING) = "Browser", "Firefox". The first one is the
instance and the second one is the class.