Good question. Earlier in Shabbos (37a) we explained in answer to another of your questions (Subject: Shabbos 37a: Rashi DH Mahu Lismoch; Date: Tue, 3 Feb 1998 19:17) that the Rambam (Perush ha'Mishnah in Kirah) describes a Kirah as being built into the ground. We said that perhaps only a movable Kirah may be placed such that the sides are above the ground and one can lean a pot against it (the topic of the Sugya there), and that is why Rashi there explains that the Kirah is movable. Here, though, Rashi is giving the normal definition of a Kirah, one that is attached (and built into) the ground.

Alternatively, it is probable that Rashi on 37a does not mean to say that a Kirah is used while it is entirely movable. The verse says that it must be dismantled, for it is attached to the ground (see Shabbos 125A). Is so, Rashi must mean that it is portable, but when the Kirah is put to use, it is fastened to the ground before it is used. It is protable in the sense that it may be unfastened and brought to the location of choice before use. If so, Rashi does not contradict what he writes here at all.

Be well,

-Mordecai