Any ideas how to troubleshoot HTTP Error 400 for MVC/Firefox?

Two clients complains on HTTP Error 4开发者_如何学C00 from Mozilla Firefox 3.6.9. This happens when they open any link - both mvc application and also any static images, which are hosted on Classic application pool without any code.

Exact error message is HTTP Error 400. The request is badly formed

No errors in windows log and application error log. Proxy in browser is set to off.

Other browsers works fine for them.

Any ideas about what could cause such behavior?

For now, i can see only one option - ask them to install Firebug, but this is not really user friendly.

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For some reason, cookies was corrupted and this was leading to HTTP 400 error. After user cleaned cookies for this website he was able to get page.

I still need to investigate what exactly went wrong and find source of this problem but it is clear that I need to review all places where I do cookie creation.