Sforno treats the non-consuming fire as a sign of miraculous preservation and as preparation for Moshe’s prophecy. The image underscores that Israel can endure persecution without being annihilated.
וַיַּרְא וְהִנֵּה הַסְּנֶה בֹּעֵר בָּאֵשׁ. דּוֹלֵק, וְזֶה דֶּרֶךְ חִידַת הַנְּבוּאָה, שֶׁהָיָה הַמַּלְאָךְ בְּתוֹךְ הַסְּנֶה, וְהָאֵשׁ בּוֹעֶרֶת בַּסְּנֶה סָבִיב הַמַּלְאָךְ, לְהוֹרוֹת שֶׁבִּהְיוֹת צַדִּיקֵי יִשְׂרָאֵל, שֶׁהֵם מַלְאֲכֵי ה', בְּתוֹךְ הַסְּנֶה שֶׁל עַם מִצְרַיִם, שֶׁהָיוּ לָהֶם סַרְבִּים וְסַלּוֹנִים, הַסְּנֶה יִבְעַר בָּאֵשׁ בְּצָרַת עֶשֶׂר מַכּוֹת, אֲבָל לֹא יִכְלוּ בְּאוֹתָן הַצָּרוֹת, כְּמוֹ שֶׁהוֹרָה בְּאָמְרוֹ "וְהַסְּנֶה אֵינֶנּוּ אֻכָּל", שֶׁהָיָה בִּלְתִּי כָּלֶה בָּאֵשׁ הַמְּלַהֵט בּוֹ. כִּי לֹא הָיְתָה נְבוּאַת מֹשֶׁה רַבֵּינוּ עָלָיו הַשָּׁלוֹם אָז כְּמוֹ שֶׁהָיְתָה אַחַר כָּךְ, כְּמוֹ שֶׁהֵעִיד בְּאָמְרוֹ "כִּי יָרֵא מֵהַבִּיט אֶל הָאֱלֹהִים", עַל הֵפֶךְ "וּתְמוּנַת ה' יַבִּיט" (במדבר יב:ח). אֲבָל מִיּוֹם מַתַּן תּוֹרָה וָאֵילָךְ, שֶׁנִּגְלָה אָז לְכָל יִשְׂרָאֵל פָּנִים בְּפָנִים וְהֵם לֹא סָבְלוּ זֶה, כְּאָמְרָם "לֹא אֹסִף עוֹד לִשְׁמֹעַ אֶת קוֹל ה' וְכוּ'" (דברים ה:כב), וְהוּא לְבַדּוֹ נִשְׁאַר בְּאוֹתָהּ מַדְרֵגָה, כְּמוֹ שֶׁאָמַר "שׁוּבוּ לָכֶם וְאַתָּה פֹּה עֲמֹד עִמָּדִי" (דברים ה:כז-כח), וּכְאָמְרוֹ "וַיַּעֲמֹד הָעָם מֵרָחֹק וּמֹשֶׁה נִגַּשׁ" (שמות כ:יח), הָיְתָה נְבוּאַת מֹשֶׁה רַבֵּינוּ פָּנִים בְּפָנִים וּמַרְאֶה וְלֹא בְחִידוֹת.
וירא והנה הסנה בוער באש, it kept burning; This phenomenon indicated the prophetic nature of the experience. Unless the angel had been within the bush, this could not have happened as something natural. The fire Moses saw burning enveloped the angel. The implied message was that the righteous members of the Jewish people, who are themselves angels of G’d, i.e. His messengers on earth, and who are surrounded by the Egyptian nation who treated them as burning thistles or less, would themselves become subject to this fire when enduring the ten plagues, but would remain unharmed by all those plagues. This is what Moses was taught when he saw והסנה איננו אוכל. it was not consumed by the flames which kept burning. The level of Moses’ prophecy at that time was not yet at the level it would be later on. The very fact that the Torah describes Moses as being even afraid to look at the spectacle before his eyes is proof of this (compare verse 6). From the day the Torah was given, as the Torah testifies in Numbers 12,8 ותמונת ה' יביט, G’d extended the power of Moses’ prophetic visions so that He shows him a visual image of G’d. The entire Jewish people had had a brief revelation of G’d’s glory at Mont Sinai only. Their power of endurance was so limited even at that time that the Torah reports them as saying that they could not even endure hearing the voice of the Lord, not to speak of enduring a visual image. Only Moses was able to retain the level of prophecy which the entire nation experienced during the revelation at Mount Sinai. This is the meaning of Deuteronomy 5,27-28 שובו לכם לאהליכם ואתה פה עמוד עמדי, “you (pl.) go back to your tents, and you (sing.) remain standing here beside Me.” This is also borne out by Exodus 20,18 ויעמוד העם מרחוק, “the people stood at a distance,” followed by ומשה נגש, “whereas Moses approached.” (to the cloud within which the glory of G’d was enveloped). Moses’ level of prophecy from that time on remained on the level known as פנים אל פנים, “face to face,” i.e. direct, not indirect. G’d told Miriam and Aaron there that He does not speak to Moses in riddles but by showing him a visual image (Numbers 12,8)