Claes,
I just tested with Firefox and was able to see it trying to download the index.php file. I downloaded the file, and in it was exactly what I showed above (only with a new user id):
{
"username": "503",
"show": "Alex Sourcecoast",
"me": "me",
"items": [
]
}Some browsers react differently to webpages that don't have the proper formatting (html, head, body sections). Chrome/Safari, which both use the same underlying rendering technology, seem to be showing that output on the screen. Firefox is thinking that you're trying to download something. The end result is the same though, some extension is trying to print out some diagnostic stuff and exiting.
I don't know what it is, but that code isn't anything JFBConnect would be doing. I'd recommend disabling features or narrowing down when the problem started to figure out what changed around then.
Hope that helps get you started,
Alex