Recent versions of Firefox have changed the location of tabs leaving very little room for double-clicking the title bar to window-shade a window. Here is a picture showing the default "tabs on top".

Tabs on top causes 2 problems:
1) If you double-click to the right of the tabs, Firefox will add another tab. The adding of tabs via double-click is default Firefox behavior that WindowMizer cannot change.
2) Because the tabs have been moved to the top of the window, there are only 5 or 6 pixels of the titlebar that are above the tabs. In order to window-shade default Firefox windows, you must double-click the area above the tabs and near the colored window buttons. This is a very small target area and it is easy to forget to click in the "sweet-spot". However, double-clicking in the correct area creates a collapsed proxy window as pictured below.

There are 2 alternative methods of window-shading Firefox windows.
1) Use the AppleScript window-shading feature introduced in WindowMizer 4.1. This option window-shades the actual Firefox window – without the use of a proxy window. This fix still has a "sweet-spot" issue requiring the double-click to occur near the colored window buttons. The results of using AppleScript window-shading with Firefox will result in a window that looks like this:

or this:

To enable AppleScript window-shading, you must add Firefox to the list of applications in WindowMizer preferences, then enable Minimize with AppleScript. The Min Height will need to be adjusted to accommodate your preferred items such as the URL bar and/or additional toolbars.
2) Use an add-on extension called Classic Theme Restorer developed by Aris. This extension allows positioning the tabs "not on top" which fixes the issue entirely. If you like the extension. please consider donating $1.99 to the developer.
After installing Classic Theme Restorer, you can set the extensions preferences to position the tabs to be "not on top" as shown below.

You can also use AppleScript window-shading with Classic Theme Restorer and adjust the WindowMizer Min Height preference to show url bar and other toolbars. The following windows are examples that can be obtained using AppleScript window-shading in Firefox:
URL Bar only

URL Bar plus Tabs

URL Bar plus Tabs and Bookmarks Toolbar

Hopefully these tips allow you to use WindowMizer with Firefox in a more productive manner. Please reply with comments or corrections. It is increasingly difficult to test every option in every app, so feedback is appreciated.