I have the same issue - some lines etc. in FF3 are missing. Hopefully Stardock releases soon an update.

OS - Win XP SP2

Here the Info from WB:

WindowBlinds version : WindowBlinds 6.1 (build 55 x86)

WindowBlinds is installed correctly on this PC
WindowBlinds appears to be activated on this PC
Your machine supports per pixel borders on WindowBlinds skins.

(SyncMaster 226BW(Digital)) 1 is attached to ASUS EAH2600 Series

Wblind.dll 2008/05/06 15:25:20
Wbsrv.dll 2008/04/28 11:35:10
Wbconfig.exe 2008/04/29 21:02:32
Wbload.exe 2008/04/28 11:35:14
Wbhelp.dll 2008/04/28 11:35:07
Wbui.dll 2008/04/28 11:35:05
Tray.dll 2008/04/28 11:35:21
Screen.exe 2008/04/28 11:35:18

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ScreenShot (you see the missing horizontal lines between the toolbars):

You might want to try uploading your screenshot to ImageShack and use the direct link to the image to post it here using the Insert Image button from the forum editor here. Accounts are free and easily accessible, if you find yourself using it on a regular basis, otherwise just use ad-hoc.

Nice screenshot, it may be I just don't know what to look for but, I can't see anything wrong in your screen shot.

Sure, looks fine. But not as it should:
Correct Rendering
Tom

No error, when using WB and the skin aerial this is proper look. aerial was created so that there are no lines between menu bars. Open any Explorer window and you will see the same look, no lines.

Edit: actually, technically they are called seperators.

Windowblinds does mess up the ff3 skin, specifically the slight gradient in colour in the toolbar, or any other toolbar theme for that matter. (I think it's because windowblinds switches the windows skin to classic)

Messes up a lot of other things too, when I try to make it switch to windows I have to use ctrl-alt-delete to disconnect my account, and even then my sound isn't working when I reconnect. But that's a whole other issue.