A lot of Firefox's bloat comes from the XUL interface. It's their cross-platform GUI which is written in XML and Javascript. It's what allows add-ons to reach in and modify some very deep elements of the browser. For example, I run the Tab Mix Plus extension which radically changes the operation of tabs.

However, XUL and Javascript is a very heavy-weight way of doing things. Chrome and other browsers have a native UI. That is partially what makes Chrome a light-weight browser. It also means that the deep level of cross platform add-on's available for Firefox will not be possible for Chrome in it's current state.

I've read the proposals for expanding Chrome extensions and it sounds like they want to go a similar route to XUL. It probably won't be as low-level because they'll want to keep their native UI code. That will help Chrome to provide some more complex plugins with their own UI but still won't allow for some of the more radical and interesting plugins available for Firefox.