Firefox blocks setting some XMLHttpRequest headers for security reasons. All or most of these are already overridden by the network layer in WebKit, but that needs to be verified, and the checks should probably be added to the cross-platform layer. See: http://whatwg.org/specs/web-apps/current-work/#setrequestheader https://bugzilla.mozilla.org/show_bug.cgi?id=302809 https://bugzilla.mozilla.org/show_bug.cgi?id=302263 https://bugzilla.mozilla.org/show_bug.cgi?id=308484
I don't necessarily agree that we should add the prohibition to the cross-platform layer. But it is indeed worth researching this. I don't really like having a P1 bug for something that might not even be broken, though.
(In reply to comment #1) I'm not sure if this counts as broken, but WebKit at least allows overriding Via (https:// bugzilla.mozilla.org/show_bug.cgi?id=302263#c5) and doesn't ignore Content-Length set on empty requests (https://bugzilla.mozilla.org/show_bug.cgi?id=302263#c17). Possibly more.
<rdar://problem/4403688>
It is also somewhat unclear how security violations should be handled in different cases (silently ignoring vs. throwing).
Setting this to P2. If we find any real examples of problems, they might qualify as P1 bugs.
A fix was committed in revision 18863 (brought in sync with the draft spec).