Our website is an ASP.net 4.0 webforms website using AjaxControlToolkit. It was online in the last week without a problem but since today one of our clients is receiving an 'HTTP Error 400. The size of the request headers is too long' when accessing the site from Firefox. It works in IE. Looking at the logs I see requests with really long junk attached after the URL:
...Statistics.aspx?_TSM_HiddenField_=ctl00_scriptManager_HiddenField&_TSM_CombinedScripts_=;;AjaxControlToolkit,+Version=4.1.51116.0,+Culture=neutral,+PublicKeyToken=28f01b0e84b6d53e:de-DE:fd384f95-1b49-47cf-9b47-2fa2a921a36a:475a4ef5:effe2a26:8e94f951:1d3ed089:addc6819: 5546a2b:d2e10b12:37e2e5c9:5a682656:c7029a2:e9e598a9
The URL should simply be .../Statistics.aspx.
I have found the problem: there were too many cookies on the site. Removing them solved the issue.
The problem had nothing to do with AjaxControlToolkit. Sorry for misleading the community with irrelevant details.
Edit: I personally have cleaned the cookies using the Firecookie plugin for Firebug in Firefox but you can clean your cookies by pressing Ctrl+Shift+Del and selecting 'Cookies'.