The issue seems to be the same as reported at https://bugzilla.mozilla.org/show_bug.cgi?id=1146297

Basically, some window managers (e.g., sawfish, twm) don't set windows' _NET_FRAME_EXTENTS, and firefox relies on this property to determine the widget position (?).

A gross workaround for sawfish would be:

(define (set-net-frame-extents! w)
  (let* ((w-dim (window-dimensions w))
         (f-dim (window-frame-dimensions w))
         (h-border (round (/ (- (car f-dim) (car w-dim)) 2)))
         (v-border (round (/ (- (cdr f-dim) (cdr w-dim)) 2))))
    (set-x-property w
                    '_NET_FRAME_EXTENTS
                    (vector h-border h-border v-border v-border)
                    'CARDINAL 32)))


(assuming the window frame is horizontally symmetric).