Copying part of a comment from bug 1571408 where I looked into this:

[It] will take a fair bit of work, especially since we have 25 <toolbar>s and the way toolbar items are handled is different (and we have many of them). Not all of those toolbars need to stay customizable, but enough of them will to make it more complicated. (Also, Firefox has already moved to XHTML for the relevant files, while we're still on XUL. That may or may not be a problem.) [...] For a more incremental approach, first convert just the main toolbar and then work on converting the others, a bit at a time. This will require allowing both systems to co-exist peacefully. [...] Just for the sake of being more precise, currently 14 of the 25 <toolbar>s are customizable="true".