I have not been able to correctly trace this error, but by only making sure that every XMLHttpRequest gets its unique name, the problem seems to dissappear. I suspect this error might be caused by an object not closing its connection properly.

[code]
Error: uncaught exception: [Exception... "Component returned failure code: 0x804b000f [nsIXMLHttpRequest.setRequestHeader]" nsresult: "0x804b000f (<unknown>)" location: "JS frame
[/code]

Curiously, this problem comes only in Firefox (I use 2.0.0.6) and not in Internet Explorer (I use ).