Firebug error: failed to load source for localhost/folder/foo.ashx on a POST in the RESPONSE tab

Firebug 1.5.4 has been correctly displaying the response returned by my webservice in its RESPONSE tab:

Console...HTML...CSS...Script...DOM...[开发者_C百科Net]

    Headers....Post....[Response]....HTML

But for some reason, now, instead of displaying the response, it is now showing an error message that I don't understand:

Failed to load source for http://localhost/myfolder/my.ashx

I'm not trying to load the source of the ashx; I'm trying to look at the Response data it's sending back to the page. There is a response (which I'm trying to debug) and the data displayed on the Post tab looks as it should.

It seems the page is posting the correct parameters to the webservice, but the response returned looks as though it's ignoring one of the posted parameters. But why this spurious error message?

Any insights would be appreciated.

Thanks

Problem went away after I cleared cache, though I don't know what caused it to starting happening in the first place -- so I won't be voting for my own answer ;-)