Does a response to an xmlhttprequest call have to be XML?

I've written a bunch of xhr stuff over the years, but today I ran into a curious thing. In firefox with firebug every time my server responds, I get a syntax error in the console. And the error context is the message response text. The message response text is a string of numbers separated by pipes "|". Sure it's not valid xml, bu开发者_开发问答t I've never sent valid xml before and never had a problem. Is this a firebug thing or is firefox really unhappy about my response text?

No. It can be xml, json, or some other structure of your choosing.