I suppose it's worth emptying the caches of both browsers, although that would be a long shot since both browsers are affected. Nonetheless, simple things are worth a try sometimes. I can see that something in 'Internet Options' might affect IE but not Firefox, which runs as a separate browser. If you start Firefox in Firefox's Safe Mode (shortcut in the Start Menu) can you see the clues then?
You might consider an upgrade to the latest version of Firefox (version 2.0). It may not help but it's best to keep up to date with it anyway.
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