Try disabling graphics hardware acceleration. Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.
You will need to restart Firefox for this to take effect so save all work first (e.g., mail you are composing, online documents you're editing, etc.).
Then perform these steps:
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If the problem is resolved, you should check for updates for your graphics driver by following the steps mentioned in these Knowledge base articles: