Firefox before 1.0 and Mozilla before 1.7.5, when configured to use a proxy, respond to 407 proxy auth requests from arbitrary servers, which allows remote attackers to steal NTLM or SPNEGO credentials.
Mozilla allows remote attackers to cause a denial of service (application crash from null dereference or infinite loop) via a web page that contains a (1) TEXTAREA, (2) INPUT, (3) FRAMESET or (4) IMG tag followed by a null character and some trailing characters, as demonstrated by mangleme.