switching between windows doesn't work when mozilla is active and open

hi i am facing a strange issue. Whenever I have multiple windows opened like windows explorer, IE, notepad, mozilla etc. I get a problem in switching between windows. Whenever i am browsing something on mozilla and then suddenly wants to switch to windows explorer or lets say do something on notepad, i am unable to do that using Alt + Tab or just by clicking the active icons in the windows toolbar. So to open other window i have to explicitly minimize the mozilla browser by clicking on minimize button and then browse or check the other windows. Funny thing is its random and not only with mozilla. Initially it was with outlook. So not sure if its windows 7 problem or is it some kind of graphic card issue.

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