Why would IE 7 display HTTP Error "Bad request" 400 on the same page that Firefox loads without complaining?
When you get a message like this turn off friendly HTTP error messages in IE. To do this go to Tools -> Internet Options -> Advanced. Uncheck show friendly HTTP error messages. Once you have done this you should get a more detailed message which will point you to the real problem.
Do you have a URL that you could share to further diagnose the problem?