What firefox version are you using? When I run it it is working as expected.
The warnings you see are from map files used for debugging to map reduced size to original positions. They are not really relevant, but also should not trigger the warning. I tested it and mine does not trigger it, but it seems the content type is missing. Are you receiving them through a proxy software that might change them? Virus software could do that for example, also I would not really like if they modify the content in any way.