Midrash Rabbah (Bereishith) explains the beginning of Avraham Avinu's relationship with HaShem. Avraham reached an epic discovery and in a vision sees a city afire. He says "Is no one in charge?" At this point HaShem speaks to Avraham for the first time and says, "I'm in charge."
To the best of my understanding the burning city always represented the perpetuation of life, consuming resources, moving onward, basically: entropy. Avraham saw entropy and said, it seems like life is chaos, but he was puzzled by this because he thought for sure there was a creator, at which point HaShem reveals himself.
Later, this time in the Humash itself, (Shemoth) we have a similar vision: Mosheh Rabbeinu sees a bush afire, and wonders how it is the bush is not consumed? At which point, HaShem reveals himself to Mosheh and sends him on the quest to free the Jews from slavery.
To me this always represented the Jewish people, they were a small sappling yet the fires of bondage never consumed them.
Finally we have a mountain aflame, Har Sinai, at the giving of the Torah. Here all the nation witnesses HaShem speaking to them directly, coprophecy of an entire people.
The humble mountain aflame with the presence of Godliness, is a pretty straightforward metaphor.
To me, on Tish'a b'Av suddenly, all three visions represented the same vision.
All of them witnessed the burning of Yerushalayim. The destruction of the temple.
At the first destruction, Avraham Avinu asks HaShem: "Is there no one in charge?" After the destruction of the Beit HaMikdash everything seems pointless. Wasn't the beit HaMikdash the very point of creation, after all, everything HaShem promised Avraham came true? HaShem comforts Avraham that no, this is all part of the plan.
At the second destruction, Mosheh Rabbeinu asks: "Why won't the tree ignite?" It seems he was asking more about the nation, why won't they burn with passion for HaShem as is there potential? (instead they burn with hate at one another, and yet they aren't consumed.) HaShem reveals Himself to Mosheh and explains again that it's part of the plan.
At the final third temple, the whole nation stands and bears witness to the oneness of HaShem, and the mountain (the Temple mount) is alight shining up to the heart of heaven.
The destruction of each Temple was a revelation of the final rebuilding, flames up to the heavens, but we weren't ready to receive it yet. HaShem's very first revelation to Avraham Avinu, the vision that began it all, was a vision of the destruction and the promise of the rebuilding.