in_place_editor_field - "No Route Matches" Error

I'm having the same issue. I think it's because I have the default
routes off, but I can't find any reference to what the route should
be. If I put the default routes back in just to test, it works.

Wait, I just checked the source. You should see something like this:

  <span class="in_place_editor_field"
new Ajax.InPlaceEditor('page_text_1_in_place_editor',
'/groups/set_page_text/1', {rows:1})

So, in your case, just make sure that /users/set_user_first_name/ is a
known route and you should be good to go.