Mozilla Firefox before 3.0.11 and SeaMonkey before 1.1.17 associate local documents with external domain names located after the file:// substring in a URL, which allows user-assisted remote attackers to read arbitrary cookies via a crafted HTML document, as demonstrated by a URL with file://example.com/C:/ at the beginning.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-1835)