Not sure if this is a known problem, but the latest beta of Firefox 4 (Beta 5) breaks WindowBlinds compatibility again. I have to tell WindowBlinds not to skin Firefox or it doesn't work properly (hard to explain here, but it's easy to see what the problem is when you try to use FF4b5 with WB).
I didn't have any problems with Beta 4 and WindowBlinds, so it must be something that changed with Beta 5.
Edit: It seems to be related to the new DirectX 2D graphics acceleration feature that's enabled by default in Beta 5. If I go to about:config and change "gfx.direct2d.disabled" to "true", then restart the browser, things look much better - not perfect, but better enough that I don't have to disable skinning.