Is this only happening in the Firefox Nightly build or also in Firefox release versions including Firefox 18 beta (Firefox 18 will be released later this week)?

• https://developer.mozilla.org/en/DOM/XMLHttpRequest
• https://developer.mozilla.org/en/DOM/XMLHttpRequest/Using_XMLHttpRequest

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

This is also happening in Firefox 17. I have already tried a clean re installation of both Firefox 17 and Firefox 20 nightly. But it didn't solve the problem. I have accidently deleted some windows files in my user folder before. Can that be the one causing this problem?

Did you try a new profile?

A good place to ask advice about web development is at the MozillaZine "Web Development/Standards Evangelism" forum.

• http://forums.mozillazine.org/viewforum.php?f=25

The helpers at that forum are more knowledgeable about web development issues.
You need to register at the MozillaZine forum site in order to post at that forum.

Yes, i tried with a new profile. But it didn't work.