Don't run CCleaner while browser window(s) is/are still open. Close all browser windows and run CCleaner twice. Failing that, clear your browsers' cache using its built-in function (Firefox -> Tools, Options, Privacy, Cache>Clear) and everything should be fine.
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in CCleaner Bug Reporting
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That worked great...but why should we have to do that? It doesn't seem to affect IE. Is this a bug that we should report to Mozilla?