Either browser caches can sometimes become (mildly) corrupted, or FFF's cache reading code is less than perfect. I suspect the answer is both.
Your best bet in such a case is to trying clearing your browsing cache before trying again (with all steps, including visiting the pages in browser).
If that doesn't help, some users have reported success manually deleting the contents of their browser cache directory; then the browser should recreate it entirely. I'd back up and quit the browser first before trying again.
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