Kommentar zu Jeschijahu 40:9

O herald of Zion Heb. מְבַשֶּׂרֶת. The prophets who herald Zion. [This is the feminine form.] Elsewhere (infra 52:7), he says, “the feet of the herald (מְבַשֵּׂר).” [This is the masculine form.] This denotes that if they are worthy, he will be as swift as a male. If they are not worthy, he will be as weak as a female and will delay his steps until the end.

The description here is of Zion herself as the herald, and so too Jerusalem. It says - you Zion the herald go up on a high mountain and you Jerusalem raise your voice with strength, don't be afraid lest the thing which you announce not come to pass, because it certainly will come. Not only this but also say to the cities of Yehudah 'here is your Gd' as if you are pointing to something which has already come and exists. (I already explained in many places that in Zion was the seat of the king, the Sanhedrin and the place of the Holy Temple, while in Jerusalem dwelt the masses. Zion announces that He has returned His Presence to Zion and the kingship and the priesthood. Therefore the prophet advised him to ascend a high mountain. Jerusalem announces the ingathering of the exiles, therefore the prophet advised her to raise her voice in strength in order that those driven away hear and be gathered from the four corners of the heavens.

מבשרת That bringeth good tidings. The feminine is used because עדה the congregation²³That is, the congregation of Zion or Israel.—This remark is directed against the Midrash explanation, quoted by Rashi, that sometimes the masculine form מבשר is used, sometimes the feminine מבשרת, to indicate that the good news will come either quickly, with vigour, as a man is used to go, if the Israelites will deserve it by a virtuous life, or slowly, without energy, like a woman, if the Israelites should fail to deserve it. is to be supplied, but not because it signifies a female person.