If you are running anything earlier than 10.6, it could be one of your internet plugins, that needs updating, or, it could also be a java issue
My recommendations (and I have seen this from time to time with customers), first, try checking for software updates. If there is a security update, a java update, or an OS updated, do that.
Next try clearing the private data in firefox, or resetting safari (under safari menu)
Stu the crazy bass from down under
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Polar bears for Obama!!