Aqua-Soft Forums...
[Help] WindowBlinds and Firefox
I have never encountered this problem as I am useing Windowblinds 5 and the same theme as well as Firefox. What you can try doing is adding Firefox to the Per Application list and using a different theme for it or have Windowblinds ignore it all together to see if it is a conflict with Firefox (which it should not). The other thing to try to do is check for any crap running like adware/spyware. Those can mess things up badly and maybe contributing to your PC's chugging.
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I had this problem.If you don't have the latest build of wb go download stardock central and update it. My problem went away after the update.