Every time i scroll down in Gmail the page truns trasnparent and i can see my desktop

Windows 7 64 bit, Current Firefox and gmail. Every time i scroll down the page goes transparent and i can see my desktop. If i wait about 5 seconds the transparency goes away but the next time i scroll it comes back. This makes it very hard for me to read what i am trying to look at. I tried turning off the transparency in Win 7 but it did n ot make any diff. While i was typing this very msg if i went past the bottom or edge of the box the letters went transparent as well. Please help i am at a loss as to how to fix this problem.