Now that mac has switched to cairo-cocoa as the default toolkit (bug 
323934), I'm going to be landing my patch that starts the process of 
switching SVG over to thebes (bug 354866).  This will be a hard 
dependency, instead of the current choice of using using cairo directly 
or grabbing a cairo context from thebes.
If you build with non-default toolkit or on a non tier-1 platform you'll 
need to disable SVG in your mozconfig.
Original warning for this change was in early October:
news://news.mozilla.org:119/peWdnYO6MaVi67zY...@mozilla.org