license for image files in default theme

Hi,

Can someone please tell me what license governs the use of image files
representing toolbar icons and similar icons that are distributed with
Firefox as part of the default theme?

Are these considered part of the source code and thus covered by the
Tri-License?

If these are not covered by the Tri-License or other open source
license, then are they subject to the license under which end-users
install Firefox (MOZILLA FIREFOX END-USER SOFTWARE LICENSE AGREEMENT
Version 2.0)?  And, if this is the case, does the license allow them
to be used only with Firefox as distributed and prohibit other uses?

I've seen it suggested that one can sidestep the end-user license by
downloading individual image files directly from LXR.  Is this true,
and what would be the license in this case?

Note, I'm specifically *not* asking about the trademarked Firefox
logo.  I'm asking about the other image files in the default theme
representing icons that appear as part of the UI e.g.
http://lxr.mozilla.org/mozilla/source/browser/themes/winstripe/browser/Toolbar.png

Thanks.