A big problem for Gecko which does not exist for WebKit is XUL extensions. If you think about it, XUL extensions must run in UI process (unlike Chrome extensions), but XUL extensions can also access web content which lives in web process. It is nearly impossible to make this 100% compatible...
I thought most Mozilla software was internally based on XPCOM. Then why can't they just proxy the interface calls to go to a different process via standard IPC mechanisms? (I've no knowledge of XUL extension model thou)
The issue is that extensions expect to access web content synchronously, so you need synchronous IPC for perfect compatibility, but that causes your UI process to block on your web process.
I did not mean that it's technically related. Rather, that it's my opinion they won't do it until then. The reason is that there is too much to rewrite/change/fix.
Then if they do manage to do it (and do it right), well, I'd have been wrong :)